My rule of thumb is that at a bar, if a beer costs less than 6 or 7 dollars, it's a $1 tip, more than that $2. Cocktails get a bigger tip, depending on the complexity of the drink, but typically land somewhere around the 25% rounded up to the dollar mark.

If you're on a tab, you tip at the end, but a slightly larger cash tip on your first drink will get you much faster service the rest of the evening.