I bought some of those WD-40 pens. These are not the aerosol spray, but rather it is sort of like a magic marker, but instead of ink, you get oil. It works pretty well, but can attract dirt. I also use a good electronic's spray cleaner that is safe on plastic, and leaves 0 residue. I got it a Fry's Electronics. I use the spray to clean the important parts regularly, then hit them with the WD-40 pen. How often I do this depends upon the flying conditions. If it's mostly inside flying, I have to do it a little less often due to less exposure to dirt.

I've done this with my little Lama 2 heli for about 1 year now with no problems. I can't tell you how many flights is has on it, but it is A LOT, and it still has all original bearings, gears, blade grips, etc, and they are still going strong.