Copied from another thread:

Quote Originally Posted by OlderWiser
As luck would have it, I'm reading a book on that exact subject. It's a book I quite highly recommend, too!

The book is Magical Use Of Thought Forms: A Proven System of Mental & Spiritual Empowerment by Dolores Ashcroft-Nowicki

It's got some amazingly good information and advice on creating, using, and keeping astral forms, and very good exercises to follow, along with great advice on many subjects. Well worth reading, IMHO.