I also have to agree about loudness. When I pass a busker, I want to be able to hear the words to the songs they are singing and not overwhelmed with loud amps.

Also, always thank a person who gives you money, no matter how little. I have seen buskers who never thank people, not even a smile or a little nod of the head to let the person know that you are acknowledging their contribution. I realize that sometimes people pass and drop some money in while you are in the middle of the song. Well of course if they leave before your song is finished, just a smile or head nod is fine but hopefully they will stay and listen to some of your songs.

Do try to wear clean clothes that look half way decent. I have seen some pretty shabby buskers out there, who look really dirty and haven't taken a shower or bath for awhile. I realize of course they probably don't have much money or too many clothes but as long as you are clean & clothes are clean and not looking like a slob would be good.

If someone has a request, try to play that song for them if you know it. People passing by really appreciate and enjoy hearing a song they have requested. If you don't know that song, just be polite but tell them you don't know that particular song but thank them anyway for the request even if you cannot play it.
It's important to be polite at all times.....and try not to smoke when you are out there in front of people. Just not the best image and many people don't like to be around cigarette smoke (2nd hand).

And don't forget to practice, practice, practice your songs before you go out there busking. Know your lyrics and songs well!
If you are doing cover songs, try to do some originals. If you know mostly cover songs try to throw in ones that are not as super well known by that artist you are covering. Try to learn some of their lesser known songs.
For example if you were doing a lot of well known very popular Neil Young songs also try to do some of his lesser known songs from his albums. I find it more interesting to hear a busker doing a combination of well known songs as well as lesser known songs.

I wish you well!