Nearly 50% of cars now sit on a pair of alloy wheels, but there are often far from glinting in the sunshine. It's hard to remember to clean what is one of the smallest areas of your car, so close to the ground and surely not nearly as important as ensuring your paintwork is flawless... but alloy wheels need TLC if they're to maintain their finish and appeal. Especially compared to a plastic wheel cover, alloys are a magnet for brake dust, which eventually gets baked on by the brake disc heat and makes cleaning with water and soap completely pointless.
We have all been guilty at some time of neglecting our cars for long stretches of time, only to get the sudden urge to give it a clean and polish and be horrified by the amount of elbow grease required. The best way to keep on top of the state of your car is to do it often. Leaving the bodywork and alloys without due care and attention leads to difficulty in polishing later and sometimes complete disrepair.
The best time to start is right at the beginning - when you apply a good coating of polish on either side of the wheels before you have them fitted to your car. This will prevent the dirt and dust from the road 'keying' to the surface and causing permanent damage and dulling.
After a week or so, depending on how much usage and dirt your car sees, you'll want to first hose off all the loose dirt and grit from the wheels (the last thing you want to do is polish it into the metal, scratching the surface) and give them a wash with mild soapy warm water. Despite the availability of many cleaners on the market, this is the best way to keep the wheels clean and undamaged. Never use wool pads or buffers, you'll scar the alloy.
A good polish is the next step, and is particularly important if you have diamond rims.
Overall, buying the best quality wheels you can, and giving them regular measures of attention is going to give you the best value for money and save you a lot of effort in the long run.