Bicycle Mechanics - Greasing cables

I started working on my older bike today. Unfortunantly it sits outside a lot because I have it up at college and have no where to put it because I use it on campus. My problem was that my rear derailer wasn't shifting at all and I found out that my cable had rusted and was sticking in the cable housing closest to the derailer. My question is what do you use to oil the cables. I just used Wd-40 because it was the only thing that I had around. It worked and the bike now shifts better then it did when it was new, but I'm sure that WD-40 is not the best thing to use. And also how often should I do this especially so this doesn't happen to my good bike that I use for riding trails.

WD40 is more of a solvent than a lube. On the opposite end of the spectrum, a thick grease may be bad also, if the derailleur has a hard time moving the cable inside the housing due to the viscosity, and so an oil may be a good compromise. For your campus bike, try using some chain lube, perhaps some TriFlow, Pedro's, Finish Line Century, etc.

For your nicer bike, if you wanted to really keep the cables lubed up, a Cable Luber (http://www.protectall.com/cable_life.htm) and a can of 3M "Wet" Silicone Lubricant make a nice combination. This thing blasts lube down the line and flushes dirt out. You could even use WD40 to flush, then use the silicone spray or any other spray lube you wanted.

Even though I am a (very minor!) WDFC shareholder, I agree that WD-40 is primarily a solvent and rustproofer. Whenever I install a new cable, I prelube it with a bit of white lithium grease. The only problem with blasting WD-40 through the housing before adding a lubricant is that the WD-40 may dissolve or otherwise degrade the lubricant. However, in an emergency situation, WD-40 often will free up a seized or sticky cable (or even chain link or derailleur) sufficiently to get you home.

Well my reccomendation is that if your cable and housing are rusted replace them. I also reccomend using a water proof lube. I use park waterproof lube. It works well and lasts along time. used it for years and years no problems at all.

I realize that as a student money is always an issue, but cables and housing are considered expendables. It is a good idea to replace them every couple of years. If you work on your bike yourself you can replace cables and housing for under $30. Most modern cables are stainless steel, and the housings have plastic or teflon liners. Of course you need a good set of cable/housing cutters. If you don't have a pair that could be another $20-30. If you can't afford to replace both, cables are only about $2 each. If you just replace the cables you can do without a special cutter. Just leave them a little long. I lube mine by getting just a small touch of grease on my thumb and finger tip then pulling the cable through a few times. You don't want the cable to be "greasy" you just want to get a very light film. While you have the cables out of the housings, you might flush the housings with WD40 to get some of the the rusty gunk out. Let them drain and dry overnight before installing the new cables.

Per Rainman's suggestion, get new cables.

When you do, spray the inside of the cables with WD-40 and let it sit overnight for the solvent to dry. This will help prevent the inside of the housing from rusting again.

When you thread the cables inside the housings, grease the cables.