Having done it, I would say it is a little like a marriage. It can be beyond your wildest dreams, or worst nightmare, depending on the others involved. Such issues as eating in the plane, scheduling glitches, liability insurance issues, apportionment of fixed expenses (equally or proportional to use during a base period) all should be addressed, understood, agreed to and hopefully signed before entering into the arrangement.
I wish that I could supply a really good link, but I remember that some popular flying magazines have dealt with this subject in recent past, and have provided sample contracts which were designed to cover most of the problems encountered in these types of arrangements.
Best of luck with it.