Well I just put up some more netting on the fence, this really helps.
Its the soft stuff that you use for plant support/protection.
Cats seem to hate this as it just gives when they touch it so they can't get over it.
I attach bean canes to the fence post using strips of plastic, then string the netting between them.
Thread string throw the top of the netting and attach to the canes, this makes a nice tight top, then just clip the netting to the top of the fence.
There are some pictures of this on http://groups.yahoo.com/group/CatControl/
I'm not bothered about cats (we have one, next but one has three and next to them one). Dogs are the bane of my life, Next door (on the other side) have four dogs, two b****y alsations (always barking), a Jack Russell which always comes into our garden to S***, and now another puppy. I tried to stop the Jack Russell by putting an electric rabbit fence up in the yard to stop him going through. At least the cats deter mice,rats, rabbits and make the Grey Squirrels wary.
We don't buy cats, they just turn and plead for bed and board (having been dumped in our cul-de-sac). The previous one used to stalk the Jack Russell, so he was very wary of her. I haven't yet managed to persuade the present one to take on this job.