I do a fair bit of promotional work for offline clients as well.
What elime says is true. NEVER do work for friends, unless you're the sort of person that likes to help your friends out for free because... well, they're your friends.
I do do work for my friends, but when they ask me to do shit for them I also make it very clear to people I know whether they're a "friend" or merely an "associate" that I have a positive relationship with.
I also establish up front with people what I expect from the business relationship, and what they can expect.
i.e. a cafe I setup a bluecaster for in Sydney wanted ongoing tech support. I said that I simply would not do anything that was not related to the device specifically. The first time they asked for something that was unrelated I just outright told them it would go to my regular fees... after a while we compromised and I took it in store credit (they make good focaccias).
I never ask for money upfront, basically because I was also working on what worked out to be a sort of CPA basis for the most part (clubs and gigs, etc), but I did have a contract and it did stipulate late fees, followed up by debt-collection services to fuck up their credit rating.
I also put in a "discount" for people that paid within 3 days of service delivery if it wasn't something that worked out to a per-person deal (aka I charged my regular rate during the "discount" period, and then charged 15% on top of that afterwards... bwuhahahaa!)
The hardest customer I've ever had to deal with was one club owner that had a few criminal connections (who I actually knew throught friends of friends... small world). It's amazing how much attitude you get from people that think they're top shit because they're mobsters. When I rang around to do some credit checking, apparently he did this with EVERYONE...
It all got resolved in the end, but it was a pain in the arse, and it's made me aware that I should do a little research on people/businesses before I start working with them.