I busked a fair bit when I was a student, & always felt that the tunes I played made a difference to how much money I made. I never managed to test it properly & scientifically (maybe I should mention I'm a statistician by trade, can't help myself wanting to be analytical sometimes...) but as a fiddle player I always felt that playing in major keys, particularly in G, made more money. No idea if that is just me, just the instrument or the tunes I know, or whether there is a cultural bias in England towards G major... worth experimenting a bit with your repertoire though & I bet anybody would find that some tunes they play are better money-spinners than others. Cheerful or sentimental tunes tend to win out.
Other things I found helpful have mostly already been mentioned - pick your spot carefully, smile & make eye-contact. Pick your time as well - depends on day of week. Weather makes a big difference - people are more generous when it is sunny. I always stood on the sunny side of the street even though it was hotter.
Finally, be as appealing as possible. Different things work for different people but as a young-ish female I always found wearing a short(ish) skirt and smiling lots helped ;) probably wouldn't work if you are a boy of course... (and take a tip from me, if you are a girl and wear a full skirt and you have both hands full of fiddle, you can't hold down your skirt when the wind blows... I only made that mistake once, embarrassing though...)
Anything else to make you appealing to kids and retired people is a good idea - they seem to be the ones who are most likely to stop and listen. Take your dog if you have one...