You would be surprised at how many people come into the shop with filthy, disgusting bicycles covered in globs of oil based messy goodness. This kind of neglect can cost money, so take the time and clean your baby. You don’t have to spend hours grooming, just 10 minutes. As often are you can bear it, give your bike a good wipe and rinse.
The most important thing is the drivetrain. Chains and cassettes get expensive and keeping them clean will prolong their life. The chain has all these little rollers, pins and plates that are all moving parts. Dirt can get into these parts and start to wear the metal away like sandpaper. Same goes for the cassette. Nasty dirty grit will eat away at the teeth on the cassette and turn them into nubs. When these things happen you will need new parts. So, lube your chain with a good lube, which creates a barrier so that dirt and crud don’t get into the drivetrain. A dry lube works well for Bay Area conditions, but if you are in a very wet part of the country, then I recommend a wet lube.
When you are doing your cleaning you do not need to soak the crap out of the chain. I see this a lot. And remember too much lube is almost as bad as none at all. It will act as a dirt magnet if there is too much. The best way to lube your chain is to drip a good sized drop onto each of those little round rollers, making sure it covers both sides. Using a controlled amount will also let you conserve some of the lube you bought at your locally owned bicycle shop.
Once you have gone over the whole chain in this manner, get a clean rag and wipe off the excess lube. The cleaner the chain looks, the better.