I think I've posted this tip before ... but maybe not here ...?
Male urine keeps cats away from your garden. If you know where they are getting in (and they usually stick to one entry point) splash some around there every couple of weeks.
Of course, if you haven't got a handy male, this might prove a bit difficult. But you could always ask a friendly neighbour ...? "No, no, I don't want to borrow a cup of sugar, but could I possibly have some wee, please?"
I used to send my grandson out to wee in my garden (which he thought was great fun). But now, at 22, he's a bit less enthusiastic!