The following is a literature review outlining the general landscape of what is currently said about habits in the personal development space. You can find more detailed information from the resources referenced below, which are cited where relevant. Preference was made to choose resources that contained the most primary sources.
A habit is a behaviour that has been repeated enough times to become automatic . Performing a habit does not require significant brain power, whereas performing a behaviour for the first time does .
Habits are how our brains efficiently navigate the world  . Our conscious mind can only focus on a few things at a time, and whenever possible, our brain creates habits to reduce the brain power otherwise required . Up to 70% of our waking behaviour are habits .
Our daily habits - whether good or bad - dictate the direction of our lives . Habits also influence our relationships and the organizations we participate in . Whether we achieve our goals depends on the system of habits that we form .
Whenever we encounter a new situation that requires some kind of action to achieve an outcome, we make a decision on how to act . We act through trial and error, and receive feedback on how effective the behaviour was . Our brain keeps track of how effective a behaviour was and experiences a feeling of reward for useful behaviours .
As the behaviour is repeated, our brain learns which situations call for which behaviours. The behaviour eventually becomes a habit. The initial brain power that was required is reduced. This process can be visualized though The Habit Loop  .
Habits can be broken down into four steps:
Cue: The cue is anything that predicts a reward and triggers the response. It tells your brain to go into automatic mode and which habit to use .
Craving: The craving is what motivates every habit. We crave the change in state that the habit provides, and not the habit itself .
Response: The response is the actual behaviour that makes up the habit, which can be physical, mental, or emotional . Whether the response occurs depends on how much friction is associated with the behaviour .
Reward: The reward is the benefit that comes from the routine. It helps the brain figure out if the particular habit is worth repeating .
The cue is about noticing the reward and triggering a craving . The craving motivates a response to obtain a reward . The reward satisfies the craving tells us if the habit was worth repeated . The reward ultimately becomes associated with the cue .
Initially, when a new behaviour is performed, brain activity is high throughout the entire habit. Once the behaviour becomes a habit, brain activity is reduced during the behaviour. This process is known as chunking, and is how habits form .
However, the brain is still active during the initial stages of the habit - when the cue is being recognized . During this time, the brain determines whether a habit should be performed, and which habit to perform . Once determined, the brain then cedes control to the habit and utilizes less brain power . At the end of the habit, brain activity rises once again. The brain assesses if the reward was received, which determines if the behaviour is worth repeating .
An important part of habit formation is the development of craving . Once a habit becomes established, brain activity rises when the cue is identified, and the brain begins anticipating the reward . The brain experiences a pleasure response from the cue - as if it had received the reward - before the reward is actually experienced . This is the experience of craving, and is what makes habits very powerful . Cravings that are not satisfied can cause a lot of distress in our brains .
Once formed, a habit can never be completely forgotten . The cue, craving and reward mechanism will always be there . However, you can change a habit by changing the routine . This is The Golden Rule of Habit Change
You can’t extinguish a bad habit. You can only change it. 
Once the habit loop has been initiated, it is very difficult to stop. However, if you disrupt the cue and never feel the craving, you can stop the habit .
These characteristics of habits are important to understand when it comes to changing habits.
Based on the characteristics of habits, we have established that habits can never be forgotten . However, the following are true for changing habits:
You can’t forget a habit, but if you avoid the cue and the craving, you avoid the habit loop 
You can’t change the cue, craving, or reward of a habit, but you can change the routine. 
Furthermore, the difference between creating a new habit and simply learning a behaviour is developing a craving that is associated with a cue which leads to a reward .
These principles guide The Four Laws of Behaviour Change, which correspond to each step in The Habit Loop .
The four laws of behaviour change are:
Make it obvious (cue) 
Make it attractive (craving) 
Make it easy (routine) 
Make it satisfying (reward) 
The more laws of implemented, the more likely that behaviour will change .
The first step that triggers any habit is the cue . Cues that are more obvious are more likely to trigger a habit . Conversely, reduced exposure to a cue reduces the chance that a habit is triggered.  That allows you to create a new habit or stop an existing one .
Habits are largely unconscious processes, where we are constantly building habits and performing habits automatically based on cues . In order to change behaviour, we must become aware of our habits . This can be achieved using The Habits Scorecard and pointing and verbally calling out your habits as they are being done .
One way to make a cue more obvious when creating a new habit is to make your intentions clear with a plan . This is known as an Implementation Intention, where you explicitly state what your cue for a new habit will be . The two most common cues are time and location .
Another way to make a cue more obvious is to stack your habits together, where you use a current habit as a cue for the new habit . It is a set of simple rules for guiding future behaviour . It is important to choose the right cue for linking the habit stack together .
An important factor is environment design . Every habit is initiated by a cue . Make cues of good habits obvious, and those of bad habits less obvious . Small changes in our environmental context can lead to large changes in behaviour over time . Eventually, the entire context surrounding the behaviour becomes the cue .
Finally, it is important to note that self-control is a short-term strategy for changing habits . It is better to avoid temptation than to resist it . Aim to reduce exposure to the cue that causes the bad habit .
An important characteristic of a habit is the craving . Habits need to be attractive, because it is the expectation of a reward that motivates us to act . The greater the anticipation, the greater the craving . The more attractive a behaviour is, the more likely it is to become a habit . Conversely, highlighting the benefits of avoiding a bad habit will make it seem unattractive .
One way to do this is through temptation bundling, where you link a behaviour you want to do with a behaviour you need to do . Another approach is to create a motivation ritual of doing something you enjoy immediately before a difficult habit . More probable behaviours will reinforce less probable behaviours .
The culture that we live in determines what behaviours we find attractive . We tend to adopt behaviours that are approved of by our culture because of a strong desire to fit in . If a behaviour results in approval, respect, or praise, we will find it attractive . There are three social groups that influence us: the close (friends and family), the many (the tribe), and the powerful (those with status and prestige) . One of the most effective ways to build a habit is to join a culture where the habit is normal or encouraged behaviour .
Habit formation is the process where a behaviour becomes progressively more automatic through repetition . Repetition is more important than time spent actually doing the behaviour . We will naturally gravitate towards behaviours that require the least amount of effort .
In order to create a new habit, cultivate an environment where performing the habit is as easy as possible, with the least amount of friction . Conversely, in order to stop a habit, increase the friction for that habit . When you start a new habit, it should take less than two minutes to do .
Throughout the day, there are many habits that occur at decisive moments, where a single or small set of choices that can lead you down the direction of good habits or bad . Ensure that the good choice is as easy as possible .
We are more likely to repeat habits that are satisfying and rewarding . Our brains are biased towards immediate rewards over delayed rewards . What is immediately rewarded is repeated, and what is immediately punished is avoided . To form a habit, there needs to be immediate feedback through reward or satisfaction . This ensures that the habit will be repeated in the future .
One strategy is to emphasize the feeling of making progress . Keeping a habit tracker is a simple way to measure whether you did a habit . Avoid breaking the chain and keeping the streak alive . Try to avoid missing a habit more than twice .
Before we can effectively build new habits, we need to get a handle on our current ones . Make a list of your daily habits, starting from the beginning of your day . Try to make this list as long as possible . Once you have a full list, look at each habit and ask yourself: is this a good habit, a bad habit, or a neutral habit ? Make this accordingly next to the habit with a “+”, “-”, or “=” .
The Goal is to pair a new habit with a specific time and location . You are more likely to follow through with a habit if you have a plan for when, where, and how you are gong to implement the habit . This makes the task simpler which means it requires less motivation to do .
“I will [BEHAVIOR] at [TIME] in [LOCATION]”
“After [CURRENT HABIT], I will [NEW HABIT].”
Your environment provides a context that acts as a cue . Prepare your environment to make future actions easier . If you arrange your environment to make cues for good habits more obvious and cues for bad habits less obvious, you can influence significant behaviour change over time .
Habit stacking can be combined with temptation bundling, where you pair an action you want to do with an action you need to do . This increases the motivation for performing the action you want to do .
After [CURRENT HABIT], I will [HABIT I NEED].
After [HABIT I NEED], I will [HABIT I WANT].
Join a culture where your desired behaviour is encouraged .
Be aware of decisive moments throughout the day and optimize the small choices that lead to the most impact .
Do something you enjoy immediately before a difficult habit .
When you start a new habit, it should take less than two minutes to do .
Avoid missing a habit more than two days in a row .
Set up automations using technology to lock in future behaviour .
Keep track of your habits and try not to break the chain .
Keystone habits are good habits which, when practiced, tend to lead to other good habits, often unintentionally . An important characteristic of many keystone habits is that they are often habits which depend on tracking and quantifying something that you are trying to improve on .