Ouch. Sorry I take care of my own.

At an open bar type of event, to be honest I tend to gauge the kind of crowd I'm attending the event with. If I'm with a bunch of people who I'm guessing aren't tipping at all, I'll probably start off with a tip of $10-15 to start then maybe go $2-3 a drink. If everyone is getting wasted and tipping, generally just the $2 a drink suffices. In the case that the original $10-$15 didn't really get me any expedited service, $1-$2 a drink is just fine.

Chris