There are some great lines out there. One of my favorites is:
"If you like what you see throw in a dollar or two. If you don't, write what I could improve on on the back of a 20 and throw it in.
That's a good one.
In his "Real Secrets of Magic" book, David Stone spends a full chapter talking about how to get tips. Some rules are not appliable to a walk around setting, but you can sure follow some.
For example he says to put in your performance as many references to money and tipping as possibile. Let just say you're going to do a trick with a bill.
"Can you hand me a small one.... like a 50. It would be a nice tip". So you underline that 50 is not much (they'll be more in the mood of giving you a 20 or a 10) and you say that you accept tips, without actually saying it.
The same thing if you need a ring "Can I have your ring? Thank you". And you act as going away. Than stop, turn back, and say "Ok, you can let me keep this as a tip if you like the trick".
Another example, which just came in my mind. You have to persuade people that you deserve tips and thath they should give one to you.
Approach someone by asking them if they wanna see "something amazing, and free... if you want". They're probably going to say yes (never ask "wanna see some magic?" people answer "no". I don't know wht, but that's what happen usually to me). Now get the deck or whatever you're going to use out of the pocket, and as you're doing this, let a 20 €/$/£ bill fall down, take it back saying something like "oh, a little tip from the guys down there. They liked the show". And you point to someone around.
Of course, you're lying, but who knows? And so they're more in the perspective of giving you some money.
As a final advice, if the spectator gives you a tip say thanks with a trick you do only in that occasion ("You were so kind that I wanna say thanks in my way" and you go for the trick) and never hide the money in your pocket. As vulgar as it can seem, make a joke about the reality of the money, and look at them in the sunlight pulling your arms away from you. Show to the people around that you received a tip. If you're going to do tricks for other people which may have followed only part of your "street show" and you're going to do a bis, they'll be more in the mood for a tip (and a magician which receive tips always deserves to be watched).
Hope I was helpful.
EDIT: I think is unpolite and "unstyilish" to perform with a t-shirt or a paper with written on it "got money friend"
