As mentioned, get some resources specific to busking; Tales From the Street is quite good, The Royal Touch is also worth your consideration. Have heard good things about To Lure With Spectacle and The Art of Krowd Keeping
What you need is a show - scripted and blocked top to bottom. People are free to leave whenever they want (and they will), so give them reasons to stay to the end. Any dead time in your show is when they'll leave, so have the transitions as tight as possible, and make sure you have something to say during these.
Work on hat lines, and put more than one in the show - Kozmo, on Tales From the Street uses three. The first one tells them you will pass the hat, the second one tells them what a good tip is, the third one thanks them for watching, and that the hat is being passed.
You need tricks that are somewhat modular - ones that people can join in partway through and still get. Also, tricks that can be performed surrounded, or very nearly so.
You have to gather the crowd first, so have something to stop the first group with; once someone is watching, others will join in. Be sure to give them a reason to stay to the end - pitch the finale often.