You can buy sprays which deter cats from your walls and fences, but if they still get into your garden, planting highly scented flowers can help, as cats don't like the pollen from these to stay on their fur. Geraniums and the like are quite good, and Coleus Canina, which flowers prettily in the summer, is another one they don't like the smell of. Apart from that, remove any wood chippings if you have them down, and replace with pebbles/stones. You could also remove any bird baths or tables, as those are obvious temptations.