I've been suckered by 'friends' who have 'given' me something and then later said 'Hey, that thing... when were you going to pay me?'

Also, I've done the same as you and helped out with the understanding that I would get something tangible in return that wasn't necessarily money.

But I will say this: A -good- friend, one that wants to maintain the friendship and not have things go sour, will offer or agree to a written contract. I have one right now with a friend and she agreed that it was a good idea to do.