A real obvious one, alluded to above, is to make sure your tip jar is placed in a visible location and is clearly and largely marked "TIPS" or the like. If you play in family places, you might want to announce a fun activity for the Kids: put a dollar in the tip jar game. If they go for it, be ready with some lines like "Stay in school kid, don't end up like the drummer here" or "Ah, here is a kid whose parents raised him/her right" or the ever classic (if the kid is accompanied by a hot mom) "Hey kid, is your big sister there going to put any in?" It usually ends up in a bunch of dollar bills from the kids and maybe the parents throwing in some bigger denominations on the way out.
I also have a word about sending someone around to collect tip money. If you are in a bar, make sure the management is cool with it. Talking up tips on the mic and even some mild over the mike "pressure" to tip is one thing, going around to people at their tables is another. I've had customers complain, resulting in getting chewed by the club manager a few times over the years. The extra TIP money you get in that case is not worth souring your relationship with the club.