There may come a time in the life of the busker when he or she may have to employ the "busker face". Essentially, this is the poker face of busking.
Example One: You are just getting into the groove of a song when you feel a piercing sting. You look around you. You have a choice to make. Stop and slap the offending bloodsucker of a mosquito that is attempting to drain you dry (once is never enough for them...), or employ your busker face, muscle through it, and tell yourself the dollars you make are worth it.
(My favorite bug moment was a couple Mondays ago when a cockroach decided to take center stage with me. I thought for sure I'd get a buck for not shrieking like a maniac when it paused by my mic stand...)
Example Two: Any time little kids or teenagers do something that isn't cute. Clearly the only option here is the busker face. No one actually wants to see you go off on some snot nosed little brat. They'll be much more impressed with and more likely to reward your saint-like restraint.
Now, one's busker face is unique to each person. It happens when you're trying to sell a CD to someone on the fence, or when you're fending off the occasional drunkard. It's in the smile you send the unsuspecting passerby who then feels obligated to tip you. It's in the moment you pretend you don't notice people staring at you and creepily taking pictures of you before sneaking off, and many other times. It grows, is shaped and then shaped again, and it is quite essential.
So if you're thinking of going down under, or even street performing, hone the craft of looking like all is easy-peasy even when you'd give anything to just go home but you haven't met your quota yet. :D