Kozmo paid for a house and an apartment (periodically) as well as paying bills and raising a family through busking. It's all about how good you are. He can pull $100 a show (which is about 20 minutes) on good days.
I will be quite up front and say this: I am not a great busker. I have recently upgraded myself from "crap" to "Ok". So this experience comes from someone who's at the ground floor - take it or leave it as it pleases you, but sometimes I find that hearing from someone who's closer to one's own level is a good thing.
you can't just perform a trick and shove a hat in their face to take money.
Yes you can. But it's better if you perform a few tricks, then shove a hat in their face.
In a more serious way - As Jordy said, do something to hook their interest, mention you work for tips, do something to hold their interest, tell them you're going to ask for tips, do the finale, ask for tips - and stand there holding the hat/box/basked/whatever while you do that.
It may surprise you how different your take can be between saying, "Toss a little something in that box over there" and standing there saying, "Please put a little something in this box here." If you can get an assistant to stand at the back of the crowd, that helps, too.
The people I see who bring in the most money, are the funniest/most engaging. A good friend of mine, Daniel Greenwolf, will do maybe 3 routines in 25 minutes. His last routine is generally fire. But he does a lot of bantering with the audience, a lot of interaction. Then he throws out the hat line, does his fire routine, and does the hat line again.
Another friend, Wyck, does a whip act. He's basically known for being crazy. He currently holds the Guinness record for most 18 gauge needles pierced through human skin in an hour (1,400), as an example. His whip act consists of him cracking the whip a few times, then telling jokes and stories for 15 minutes, then hitting a target held in someone's hand, then 10 minutes of talking, then the big trick. I've literally seen him do a half hour show with everyone enjoying every minute, and only do one trick. He makes a few hundred a weekend usually.
So, be engaging and fun. Don't be shy about asking for money. Make sure you tell them before you ask for money, that you are going to be asking for money. Make sure you point out that what they have experienced has value. Stand at the front of the crowd and ask for money. Pack up/get ready for the next show after they disperse, not while they could be giving you money.
And my last bit of advice: You'll get the most money from a short, super fun show, rather than a longer show that has less strong/engaging material. Some of my best takes came from 20 minute shows where I just bantered with the audience a lot (while still doing my routines).