One of the most effective exercises of astral projection is concentration on the heart. This exercise has been used for this purpose for probably thousands of years, and it’s the one I’ve used most. Virtually anything that focuses the mind upon one thing can be used to project with, but concentrating on the heart is a good basic exercise to have and it’s an efficient technique to learn to astral project with.
The heart is not only a vital organ of the physical body, but it also has spiritual aspects, which is why it has so many references in spiritual art and literature. One of these aspects is the heart chakra, another is the divine firmament, another is the heart temple, and there are many more.
Concentration on the heart can be practiced at any convenient time without it being just an astral projection exercise. But like any exercise that uses concentration, you’ll need to put in quite a lot of practice and gradually train yourself for it to start to work.