You tip when you pay, whether you're running a bill or buying drinks one by one.

If you're paying by card, usually the little card-swipey-machine(?) will ask if you want to tip, and how much. Nice and easy.

If you're paying cash, you can drop some into a visible tip jar, or leave a little pile on the bar/table. It's convenient to overpay and then use some or all of your change for this. You don't need to stick around to watch this be picked up. edit: absolutely agree with JoshuaZ- you should wait for your change. After accepting it you don't need to be present when the bartender gets the tip.

Sometimes, more in semi-classy restaurants, a waiter/ess will ask if you want change- if you say no, the difference is tip.