I've been busking the last 2 weeks or so.. it's actually going pretty well..
Old People LOVE the ukulele. So play songs that your grandmother would like.. love songs are normally good. We play Ingrid's The Way I Am when we see a group oh older people coming our way, especially couples.
You kind of have to play for what you say, have a few different kinds of songs in your arsenal. If I see a group of kids my age.. i'll blast some black eyed peas or paper planes or buy you a drank. J Holiday - Bed is a good one too...
I'm Yours, Plain White Tee's I use pretty much whenever i'm bored or don't know what the people will like..
Little kids with their parents are $$$ they love putting money in our tipjar.. we made a tipjar out of some stuff we found at walmart.. go to my video on my youtube, i think it's the new black eyed peas one we made and I show it in that vid.
Basically.. know your audience,
Location, location, location.
You want to find a spot where THEY HAVE to pass by you. Not that your too pushy on top of them, but they have to listen to you. Where we normally sit, is a bench by the ocean. It's sits right next to the curb where EVERYONE walks by. If you can get a wall behind you, that'll amplify your music so you won't have to play or sing as loud.
A park can be sometimes hard to find a spot like that, depending on how it's built. Where I play there are restaurants, candy shops, lots and lots of tourist stuff, I dont know i'm in a pretty lucky spot down here.
If I can think of anything else I'll add it.
Hope this helps a little.