Me it's $1 for a $1-19 order, $2 for $20-$29, $3 for $30-39, so on and so forth.
I visit this redneck-themed burger place every few days and since the only thing I ever order is $6.20, I feel it's appropriate to tip $1 each time.
This one time I went out of town and ate at a locally owned Pizza joint and I wanted to tip the person who served me but couldn't think of a way to do it without being patronizing or awkward, so I didn't.
How do you tip? When I tip at the place I mentioned earlier I just throw it in with my payment, since I pay in exact change plus the tip, there's no awkwardness.