I get asked this a lot, and I've never sat down and typed up a real good FAQ answer, so here goes.
How should I maintain my (steel) bike frame?
So: lots of things conspire to destroy bike frames of all kinds (ie crashes, running into your garage, big jumps that you didn't mean to go off of, etc) but the one that most folks worry about with steel bikes is rust, so that's going to be the main thrust of this post.
First, remember that most modern frames (including everything I make) are some variant of "Cromoly" steel. That means that the steel (iron and carbon) has been alloyed with Chromium and Molybdenum. Older bike frames (say, from the 1970s or earlier) tend to be "Hi-Tensile" or "Mild" steel, both of which are just basically iron and carbon. Chromium helps steel resist rust (if you alloy with enough Chromium, you have stainless steel) so modern frames are a lot less sensitive to moisture than older ones.
That said, if cromoly gets wet and stays wet (especially if the water is salty) it can and will rust.
To prevent rust in a steel bicycle frame:
-Apply framesaver. This is available at any good bicycle shop. If you can't find it, you can also use Boeshield T-9 (essentially the same stuff, available at most hardware stores), which is intended to prevent rust in airframes for steel framed aircraft. If you want to be a retro grouch or impress a hipster, you can use boiled linseed oil (bonus: good for grooming your mustache, too!)
-To apply (assuming you are starting with a bare frame), just spray lots of the liquid into the vent holes and water bottle bosses, and down the seat tube, then find a spot in your garage where you don't mind some spills and swish the liquid around inside to evenly coat things. Keep in mind that it'll keep draining out of various places for a day or two, so don't put your bike in your Audi or on your mom's carpet.
-Waltworks frame owners: note that I do this before I ship your frame out, so you don't need to do it yourself.
-Framesaver isn't a magic bullet, though. It slowly loses effectiveness over time, and if water is allowed to just pool inside the frame, the framesaver can't prevent damage.
-There are really only 3 places in a typical frame where water can collect: at the bottom of the seat tube, the bottom of the downtube, and (on frames with vent holes at the dropouts or bottom bracket) the chainstays where they join the bottom bracket. The BB shell itself can also accumulate water but that's typically not much of problem since you should have greased the threads when you installed the BB, and the shell material is pretty thick. Note that drilling a vent hole in the bottom of the BB shell will let water drain from the BB - but it can still accumulate in the bottom of the seat tube, down tube, and chainstays.
-Waltworks owners note that in all frames built since 2009, I've been completely sealing the rear triangle, so you don't need to worry about the chainstays.
-Since those 3 spots are the areas of vulnerability, your goal should be to prevent water from accumulating there and staying there. That usually is pretty simple - after a very wet ride or every few months, take the seatpost out of the frame and flip it upside-down. Leave it in a warm spot or in the sun for a few hours, and you've dealt with the water. Regreasing the seatpost can be a good idea at the same time.
-Once a year, strip down your frame, make sure the inside is dry, and regrease/reinstall components.
-Once every 3-4 years, reapply framesaver (more often if you ride in the wet a lot, less often if you live in the desert).
-Of course, you should keep your bike inside out of the elements, too. If you take even basic care of your frame, it should last a long, long time without any rust problems.
And a final note: make sure you keep your chain from dropping off your cassette onto your frame! Limit screws are important folks - don't let this happen: