In reply to Timmd:
I have been told of two options and can confirm that one had a 100% success rate.
The first one is to get some lion poo from a zoo. I've heard it works.
The second involves throwing scalding water over the cat. I know somebody who did this and confirmed that the cat never came into his garden again.
I just let my dog loose into my garden and watch the cats run as fast as their little legs will take them. Its either that, or let the cats eat the birds which feed in my garden.