I almost always leave the counter person a buck or two on a pickup. More if it is an extravagant order or I have asked for special services (on the side, extra hot sauce, make sure they are crispy, etc.)
At a bar, if you're just drinking then I tend to tip on my own personal inexact formula that considers # of drinks, time spent at the bar, service and conversation with the barkeep, etc.
If I'm eating, too, I might use the % calculation (especially if the entire bill comes on one check). If that's the case, you don't have to consider the regular price of the drinks (although that's a good touch if you actually know the prices), but keep in mind the bill is low and be generous with the %.
In general, I figure out what a good tip should be (20-25%) and then add another buck or two (or more depending on the circumstance). It makes a difference if you are a return customer, trust me.