Whether it's your nice furniture or the backyard garden, there are some places you just don't want your cats (or the neighbourhood cats) getting into. A little vinegar can help with that.
Put some vinegar in a spray bottle, and use it on the areas where you don't want cats. Most cats will elect to find somewhere else to sit if they detect the scent. The vinegar also has the handy added effect of being a decent deodoriser.
Vinegar As a Cat Repellent [eHow]