ZinZan has an interesting idea with % reduction everytime you cash.
I think the big thing to keep in mind here is that it doesn't so much matter what the agreement is as much as it matters that you both agree to and abide by it. Friendship is great and backer/sponsors helpful, but business and pleasure do not mix (nor a good combination make). Remember the old saying that there are no friends at the poker table? Well it similarly applies here too.
I don't object to taking a sponsor, but I would highly suggest that any type of deal get put in writing and signed by both parties. Make sure ALL details are spelled out on who get what and how, etc. This way no one gets slighted and no feelings are hurt. Case in point, look at Jamie Gold and the stupidity that lead to his current situation (based on the details that we know). I know that this wasn't a friend that he got into it with, but a written contract with all obligations, payouts, etc would have been huge right now.