Tipping the Sidewalk Vendor

Since this is more from a customer point of view than a vendor point of view, I'm starting this thread here rather than in one of the professional's forum.
 
What is your tipping policy with street vendors?
 
I admit that I never tip a street vendor, unless I ask for something unusual. For example, at a hot dog stand near a post office where I used to live, I would occasionally make an order to go, but ask to pick it up on my way back from the post office. I always paid in advance, and  gave the vendor an extra couple of bucks (this was an order for several people) in advance as well. Truthfully, his prices were so good that even with the tip, it was still a bargain (and this was one of those guys who, if you asked about his food, would proudly go into great detail; as I have stated in other threads, I usually take this as an indication of quality).
 
Bart