In reading books, taking courses, and practicing Astral Projection there seem to be 4 overall key points in reaching the out-of-body state.

CONSISTENT PRACTICE
You should aim to get into the habit of practicing astral exercises, and be dedicated to them. This means being disciplined and and focused on spiritual development and astral projection in whatever activity you’re doing. Having dream reminders, reading astral books, interacting with spiritual groups (online), and activities to keep astral projection fresh in your mind.
It’s also important to prepare the groundwork for projecting into the astral plane. If you can get the preparation right it will be easier to project and you will be able to go out of your body much more often. Without preparation the techniques to project are less likely to work. The period of building skills for astral projection is similar to acquiring other skills, like learning an instrument.
One of the first things to do is to prepare your environment and surroundings for astral projection. Make sure you are clear of possible distractions(phone, tv, lights). Create a quiet environment that you are able to take time to focus on practicing. Consistent exercises in concentration and visualization will greatly assist both beginners and lead to more experiences.

RELAXATION
It’s important to learn to relax the body for astral projection to take place. If you’re tense it will be more difficult to focus upon the exercise. There are techniques that can be done; it’s a matter of relaxing the muscles in the body, a deeper meditation. Exercises of relaxation prepare you for the stages of astral projection.
Here is an example of a common relaxation technique:
Spend about 10-15 minutes doing this exercise.
Relax the body by tensing each individual muscle slightly and then immediately letting it go loose. Go through the whole body this way, paying close attention to parts of the body that could be tense.

FOCUS
To successfully project it is very important to be able to focus on the exercise of astral projection you are doing at the time. In other words, mindfulness. You shouldn’t be distracted by other thoughts or by anything else other than what you are focus on (which is usually your breath). To do that, train yourself to concentrate and visualize before you begin your exercises and after you have relaxed. Make concentration and visualization exercises part of your daily routine. It will also keep your mind sharp, focused, and more ready for projection. Willed astral projection usually occurs when the mind is concentrated on projection at the exact moment of mind awake / body asleep.

VISUALIZATION
You can also use visualization for astral projection and preparation for it. Visualization is using the mind to consciously imagine or remember something with dimensional structure. This visualization technique is also demonstrated in remote viewing. The less the subconscious can analyze the subjective elements, the better you can visualize. You can train yourself to concentrate on something that is your thought entirely within your imagination. Then using that imagination, mental focus and preparation to achieve an astral projection.
TO RECAP:
oo Learn to relax your body. It takes a short time to learn to relax and then train yourself to concentrate for an astral projection
oo Start with short daily exercises of concentration lasting five to ten minutes, and gradually increase the amount of time to twenty to thirty minutes as you feel comfortable with it.
oo Relax the body completely before doing a concentration exercise.
oo Begin your exercises of astral projection whenever are ready but maintain daily concentration exercises whenever you are doing nightly astral projection ones.
Good Luck!