Ive never been good at this either but I always try to mind map everything. Specifically what the character looks like and almost any information about them. eg family. age and all that. home town. beliefs. Morals and personality. basically everything.
Then I mind map what I want the character to be like at the end of the book or story. Then If I really want to mess with them I mind map what I want them to be like in the middle if something bad happens or whatever. Then I can decide on what forced this change whether good or bad.
Thats what I do, not very well but I try. lol
Hope it helped!
Aidan