Fencing: Although most cats can jump most fences, there are some methods that might help keep them out. A tight string or a row of spikes along the top of your fence (commercially available to keep birds from perching, or try purchasing carpet tacking strips) may keep them from walking along the top. Chicken wire bent to face away from your fence can keep them from getting over the top, and strips along the bottom of the fence, or between any gaps, may keep them from getting under or between.
Mulch: Loose, bare soil just calls "Potty time!" to cats, so keep it covered. I used dry leaves this fall, which our neighbour's cat simply dug up. However, something harder to walk on, such as pinecones or rosebush or raspberry stems, might make a cat think twice about invading. Laying down chicken wire is also sometimes recommended as a surface they don't like to walk on, and as an added bonus, they will keep the squirrels away from your bulbs.
Repellents: Several herbs emit distinct odours that cats are said to dislike, including sage, rue, lavender and pennyroyal. Coleus canina, a variety of the common ornamental annual coleus, is also supposed to repel cats, dogs and foxes by its smell. Citrus peelings and coffee grounds scattered around the garden are also sometimes recommended to repel cats. There are also several commercial repellents available at garden centres, often using sometime of predator urine to scare cats away.
Attractants: Planting catnip in a remote part of your yard might keep cats from digging around your more choice plants, but why attract them to your yard in the first place? Less ethical, but more effective, might be to make a gift of a catnip plant to a neighbour.
Motion: Strips of tin foil or old CDs that flutter in the wind and reflect light might scare away some cats, but likely won't be effective once they get used to them. Some people even claim that a clear bottle half full of water will scare cats because of the way the light refracts through them.
Noise: My kids like to bang on the window whenever a cat comes in our yard. This doesn't always work, but they usually run away if we open the door and threaten them. Motion-activated singing frogs can also be effective in your absence. Some products emit high-pitched noises that are supposed to repel animals, but these haven't been proven to be very effective.
Water: A very effective deterrent (which I haven't personally tried) is a water pistol, and one dose might be enough to keep the cat away for good. My brother informs me, however, that your cat-owning neighbour might not be impressed when they catch your three-year-old son chasing their pet with a water gun. Motion-activated sprinklers are easier, and are also effective against larger animals, such as deer and teenagers. Just make sure to turn them off before you go into your own yard.
Unfortunately, most of these methods have pretty low success rates, but keep trying until you find the one that works!