Step 1: Tools of the trade
The purpose of the sponge is as follows; the sponge is porous, so it holds water. Rubbing a hot soldering iron tip on a wet sponge causes the solder to contract at a different rate than the soldering iron, helping to knock off any globs of solder that may be clinging to the tip. NOTE: Make sure the sponge is damp (not soaked) with water.
600 Grit Sandpaper
The second tool of the trade, is 600 grit sandpaper. NOTE: Paper ignites at 451 F, so make sure the iron is unplugged, and has had time to cool before using sandpaper. You will only use sandpaper if the tip has been abused by the previous technician, student, or co-worker.
Tip Tinner / Cleaner
You wont need this if you've got some extra solder. I wouldnt recommend spending the money to buy it unless you have a high-end soldering iron ($XXX.XX price range) for all intensive purposes, regular solder will work just as well for what we're doing.
Step 2: Cleaning the soldering iron
Scenario 1: Someone left me with a cold & dirty iron.
This is common in some workplaces / college electronic labs, etc. If the tip of the iron is covered in gunk, it may not heat properly, even if you heat it up, and use the steps listed under Scenario 2. If this is the case, unplug the iron and allow it to cool. After the iron is cool, lightly scuff the surface of the iron tip with 600 grit (or higher) sandpaper until it begins to gain some luster again. You are not trying to remove metal, just the oxidation. (notice the tip of the iron in the provided picture)
Scenario 2: My Iron is dirty from use, but still hot.
Remember that sponge from earlier? All you need to do is set your iron to the side and allow it to heat for a few moments (about 90 seconds is usually sufficient). Once the iron has heated, you'll start to notice some brown deposits on the tip. This is rosin. Simply take your iron, and flick the tip on the wet sponge (WARNING: do not hold the sponge in your hand to do this.)
We're almost done.. but not quite. Procede to the next step.
Step 3: Tinning the tip
To tin your tip, use a tinning compound, available at RadioShack. If a tinning compound is unavailable, you can substitute this with regular electronics solder. I recomend a low temperature solder for this because you want the iron to cool fairly quickly, so you dont fry the solder onto your tip which would defeat the purpose of cleaning.
A properly tinned tip will look similar to the tip below. An instructable has been created by royalestel demonstrating the proper technique of tinning a iron tip. The instructable can be found here.
A properly maintained soldering iron will give you years of flawless service.