You must be able to concentrate and focus completely on what you are doing. Lack of concentration is the single, biggest cause of projection failure. It affects every aspect of projection, from the relaxation exercise, to the trance state and actual projection.
To test your ability to concentrate: Sit and relax. Close your eyes and clear your mind of ALL thought. Breath slowly and deeply and count each breath, at the end of each exhale. Hold it totally BLANK, apart from the counting, for as long as you can. See how long you can hold it like this. Be honest with yourself, every time a thought intrudes start counting from the beginning again.
You are doing well if can do this for longer than ten breaths. Ten breaths is, however, not long enough. Don't worry though, this can be improved on with the exercises below.
Concentration Exercise (1)
After image retention: Relax, calm your mind, and look at a candle or a light bulb. Place this light in front of you, a couple of feet away, and stare fixedly at it for a minute or two. Close your eyes and concentrate on the after image this will generate behind your closed eyelids. Try and keep sight of it for as long as possible. Use breath awareness to keep the mind clear, while you are doing this. Try and make the after image grow, instead of fading away.