Busking is street entertainment, so remember you have to actually entertain people. To be a professional busker learn popular songs and involve your audience, getting them to sing along and join in.
Try incorporating a gimmick into your act to encourage participation - perhaps a short song about the town or resort, that everyone can join in with.
It's possible to make a good living out of busking in the summer if you can find an area populated by tourists and/or young people and do a regular performance. After a few days you'll begin to build a regular following.
You could ask a friend to circulate among the gathered throng accepting donations while you entertain.
It is a good idea to contact your local council and enquire about a busking licence. In some popular areas a licence must be purchased in order to perform. Some even hold auditions!