Cats hate the smell of oranges so put pieces of orange peel in areas that they frequent. Smear your fence with orange oil. Choose big oranges and squeeze the skins with the outsides together and the oil will ooze out!
Squirt the little intruders with a water pistol, cats hate getting wet and won't come back if you aimed at them a couple of times. It may sound cruel but it has worked in my garden.
Get some thorny twigs, eg; rose, berberis and lay them on the ground all over the garden. They don't look unsightly and are cheap and easy to replenish.
If you don't have any cats yourself there is a wonderful gadget you can get from garden center's or DIY stores that makes a noise that only cats can hear, they don't like it so avoid the garden. You can get it mains or batter operated. Its been the only thing that has worked for me.
If you have cats yourself then the above it not suitable.