Always, always, always sign a contract!
This is the one tip you shouldn't miss regardless of what type of business you have. It doesn't matter if you're working with a friend/family member or doing a barter or some sort.
A contact will outline both your expectations without putting a strain on your relationship. So it may be awkward at first asking your bff to sign one but trust me it saves you from far worse situations.
Think it's too much work? Think again, I have a standard contract with everyone where I just adjust certain components to fit their needs. This way it's not adding to my time or taking away from precious design time.