Try a Water Pistol - I have a super soaker which I shoot at the local cats when they come in the garden or venture on to the front lawn. It works great as the cats hear the noise of the water coming out & shift fast. If you should hit them they move even faster (i got my neighbours cat the other day and it cleared a 6ft fence in one go - nb my neighbour approves as cats hate getting wet, and he regularly tips a bucket of water over one of the local tomcats). Mind you I still get presents of eviscerated mice on my front doormat (maybe they aren't being friendly - cat costra nosta?)
One over friendly cat got hit full in the face when it thought I was opening the patio doors to let it in... I nearly missed the follow up shot as it turned tail. I haven't seen that cat in my garden for 6 weeks now, and it keeps its distance if we meet in the street.
Picking them off the fence with it is also fun (particularly when they fall inside the garden & try to escape).
Anyway the trick is be indiscriminate and shoot wildly as having several shots increases the terror (nb try to make the 1st shot a hit or a near miss). A direct hit seems to be good for at least four weeks.