Just keep trying to get it back. Don't resort to violence, but tell your friends so that they don't lend him money as well. Chances are, he doesn't have it and he's embarrassed that he owes you money. This, let's say friend, but I regard him the same as you regard your friend, owed me about $85 for almost a year, and when he finally paid me, after being nearly impossible to reach, he said that he was inbetween jobs a lot and was just trying to get by, and wasn't sure how I'd take it, so instead of own up to me this entire year, he decided to avoid me and just pay me when he could. I haven't seen him since then, I think it really embarrassed him but I told him not to worry about it.
All in all, let your friends know so they don't get in the same boat as you, and maybe try and be sensitive to why he hasn't paid you. Unless he's just gypping you, in which case f**k that guy.