Here's a really good video and comprehesive FAQ that explains all about soldering (and when and how to tin your iron).

A much cheaper option than the metal shavings coated in flux, is to put a pan scourer like this one:

in a small container, and use that to clean the excess solder off of the tip, it seems to do a reasonable job and doesn't cool the tip excessively. Although you don't get the benefit of the flux which coats the shavings either.

(A tip: make sure that the scourer isn't plastic!) ;-P

(grrrr... not enough rep to comment) I used to wipe my tip on a wet sponge after every couple solder joints. You definitely don't want it to look dirty. This retinner can be used if things get really bad (it seemed like that happened every couple hundred joints). If you don't want to buy that, then just melting a bit of resin core solder onto the tip works almost as well (but adds its own dirt...).

More recently, I've been using a brass sponge on a temperature controlled soldering iron. I haven't needed to retin at all. I don't know if it's the better iron or the brass sponge. In other news, I'm completely in love with that soldering iron. It heats up very quickly and then stops without getting too hot. The tip doesn't show any wear, despite quite a bit of use. One of the best purchases I've made in this hobby.

My soldering station has a sponge, a small steel wool pad and a little block of very fine grit sand paper.

The sponge and steel wool explain themselves. The tiny bit of sand paper is for the odd instance where I accidentally come in contact with some kind of plastic or insulation after using flux .. which the steel wool just seems to roll around on the tip and the sponge won't catch.

The tip on a good iron (i.e. Weller) should last for years if taken care of properly, what kind are you using?

I think it can depend on what type of iron/tip you have. I've got a ceramic tip and the manufacturers instructions say not to use an abrasive to clean it. I use a wet natural sponge to clean off dirt whilst soldering, and I tin the tip with pre fluxed solder after every use, when it gets dirty I clean it off with the natural sponge, then leave it to cool down. Once cooled I use isopropyl alcohol and a cloth to polish the tip. Normally I'd clean it in this way once a week, and I use my iron daily. Over all this method seems to preserve the tip for longer I've been using the same one for nearly three months now and it shows no signs of letting up.

So I'd say the first port of call for finding out how to look after your tip, is to contact the manufacturer and see what they recommend.

Metcal recommends de-ionised water on the sponge, but I don't bother. Most of my cartridges have lasted for a couple of years, which is a good thing as they are rather expensive.

They supply a brass cleaner with their new MX-5000 system, so that is probably the thing to use with other systems.

You should tin the soldering iron whenever any part of the tip does not have a shiny appearance.

Flux is a good tinner, so if you don't have soldering iron tinner then just keep melting solder with flux onto the tip wherever it's not shiny and let the bubbling flux do its job.

I use a brass pad type tip cleaner rather than a sponge - water will corrode the tip quickly, so I've completely moved away from damp sponges.

I also add a little solder to the tip every time I place it in the stand (and wipe the excess off when I pull it out) this puts a layer of solder coating the tip so it doesn't oxidize in the air while it's sitting there waiting for me to pick it up again. When I turn it off, the solder blob stays on the tip and protects it long term when not in use.

Since starting this technique I haven't had to fully re-tin my tip in years, and my tips do not develop the pitting and corrosion I used to experience.

Keep in mind that to solder some things it'll heat up a little faster if you keep a bit of solder on the tip, and touch that to the part and PCB pad - the solder blob will transfer heat more effectively than the bare metal will. If the tip won't hold solder, it won't heat the joint effectively.

This is going to sound kind of basic, but it took me a while to figure out that I had to occasionally pull the tip out of the iron, and clean out the socket and the back of the tip. Get enough rust or other junk in there and you'll never get good heat transfer from the iron's heater to the tip itself.

I use Multicore Tip Cleaner. It's solder powder mixed with an aggressive flux and does a very good job with my Metcal cartridges when they need tinning. I just poke the tip in the cleaner and wipe it on the sponge. My cat seems to like the taste of the stuff so I have to keep it covered when I'm not using it, otherwise he licks it, which isn't very good for him.

If you're holding the solder to the tip and it's not melting it's because 1) the tip's not hot or 2) it's really dirty. Both are things worth checking.

Other answers talk about the range of options to clean the tip, all good, all dependent on your particular tip. The point of all of them though is that you don't want to score the tip, you want it clean and intact, without pitting or scrapes, so be gentle. In nearly all cases, the tip will be hot when you're cleaning it.

A trick I learned from an aerospace tech is to use a lot of solder when I clean the tip, using the excess solder to almost 'flush' off the tip. Flux or Rosin is your friend to help distribute heat, but don't leave it on, as it will corrode. Alcohol cleans rosin up nicely.

Just as a note too, you'll buy "flux", but make sure it's a "rosin" flux. There are many kinds of flux, and the rest are unsuitable for soldering irons and pcbs. They might work in the short term, but cause endless headaches in the long run with corroded parts.

Sometimes when it takes an iron a long time to melt solder, the problem is that oxides have built up in the thermal joint between the heating element and the tip, reducing the ability of the element to keep the tip hot. This is easily remedied by simply loosening and re-tightening the screw or collar the holds the tip in contact with the heating element.

With gun type irons, there are usually two set screws, one for each leg of the tip. In this case, the tip is the element, so the oxides present electrical resistance, but this accomplishes pretty much the same lack of heating. Just backing the set screws off half a turn and re-tightening is usually enough to restore the heat to this type of iron.

Of course, it doesn't hurt to periodically take these things apart, and clean them up with a small wire brush, either. I use the same brush for this that I use to shine up component leads and surfaces before soldering them.