Good Rule Number One, there, Glenn: stand instead of sit, absolutely! (Unless you have a piano out on the street, or some other instrument at which you need to be seated).

And, rather than "Perform TO your crowd," I'd rephrase and say "...TO you audience," even when there isn't a crowd. Keep in mind that even those people who are walking by can hear you, and may have been listening from a block or more away, since before you ever saw them. So try to make eye contact with anyone and everyone you encounter; you may be surprised at who's actually interested, and a smile and acknowledgement may be all a person needs to decide not to keep walking but to stop and listen for a bit, and/or to dig out a bill or a few coins after all.