I did actually did this once after stopping and observing a bum and wondering this same thing. I talked to him for a few and he told me it's easy. You just stand there with a sign. That's it.
He told me that with my looks, (young, handsome, "kid") I would do great. He said the women would give me a lot of money. So, forgot to mention, I was pretty drunk so I was like "sure wtf, time for a little research". He handed me his jacket (so I looked the part). I went to the spot he told me nearby (off-ramp) and simply stood there with a his sign that read "Hard times. Please Help."
I got 80 dollars in about 30 minutes. A whole 20 from some desperate housewife I could have easily taken home from the bar, if that were the scenario...
Was very eye opening... I have never done it since, but I always wonder "what if I had stood there all day..."
OH WHICH REMINDS ME: The other day I needed change for a twenty for bus fare (planning on downtown drinking), and while out in the parking lot, I saw a pizza delivery person. I was like, "well instead of going back into the store, I bet this person would have a million singles".
I actually half flagged this lady down and started to explain that I needed change for the bus and she looked like the right kind of person. Then, she starts to give me a ton of change and I was like, "No I need change for a twenty"..
She looked at me kind of funny and then said, her last delivery cleared her out of small bills. And I said thanks and was on my way..
It wasn't till a couple of minutes later that I realized I had gone on for a moment about "change", never mentioning that I had money on me. She thought I was pan handling AND this chic was entirely ready to hand me bus fare. LOL. I seriously did LOL.