Loans and Waitlists

Forgive me if this has been covered already- I searched the forum but found nothing on it, so here goes.
I have been accepted to School A and just got my financial aid package info. I am also still waitlisted at School B. So: what is the deal with waitlists and loans? Do I need to wait until I am sure that I am going to School A before I accept their aid- or can I go back on the aid acceptance if I get accepted to School B? Both methods seem to have potential problems.
This is probably a really obvious question to some of you, but all this stuff confuses me!