I usually smile at people and say "Sorry!" while hurrying away. Who knows how sorry I really sound, but it is a strategy that has worked awfully well. When you're in a well-traveled area, canvassers and panhandlers alike know their odds are much better with someone they can trap into a conversation than someone that puts up a "oh I am terribly sorry but I don't have a second to spare! really!" front.

Don't forget to keep walking! Sometimes I even put my hand up in a "I'm going to stop you right there" kind of way. I used to ignore panhandlers but I've found that (around here at least) you get a much better response with a sad smile and "sorry" than a "you don't exist to me" attitude. I guess it's more that it makes me feel better rather than it being any different for them. As much as I'm not going to give them money, I hate to pretend that people aren't worth acknowledging.

Canvassers are easier to be short with, especially if you don't care for their cause. How about, "I gave at the office"?