As Mograt says, get some manure from a big cat and bung that on the garden. Your best bet for getting that is 'Zoo-Poo' which contains a mixture of nutrient rich poo and tiger / lion poo (which contains almost no nutrients as their digestive system is so efficient) - the best of both worlds.
Another thing which I once tried was a technique which I have also found to be good with moles (providing you have understanding neighbours who don't object to you owning a singing lawn). Get a musical birthday or Christmas card and take out the musical bit. Set off the mechanism, bung it in a tube and stick it all in the garden. It seems to work well on the cats but eventually I got so sick of hearing the garden singing 'Happy Birthday' that the plan was scrapped. Anyhoo, if perhaps you are somewhat more resilient to that kind of thing perhaps that would be worth a try. (Personally I would go with the zoo-poo, but perhaps I'm a bit biased, being a zookeeper myself). By the way, if ever you do use the music thing on moles be prepared to have to keep reburying them, the moles keep chucking them out of their holes for the first week or so.
As a final note, bear in mind that most of these cards will last for absolutely ages. If you've buried the things they just won't shut up soon enough.