When you find out which songs work, keep playing the same ones over and over. You can get by with about half a dozen, if you don't get too bored with `em. You're audience will likely keep changing. Also, try and learn a couple of the hits now, even if they sound pretty spare with just a guitar (or whatever you're using). People will appreciate that and just want to hear the song they really like right now, no matter how far you get from the recorded version.

Also, a duet (or more) busking seems to be taken more seriously than a solo and can make more money, though you will have to split it of course. You'll probably come out further ahead that way.