Going through Soldering tips quickly
One thing I haven't seen mentioned yet is this: Whenever you are planning on not using the iron for more than about a minute, load the tip with solder. This way, the solder oxidizes in the air, instead of your tip. An unprotected tip will start to oxidize pretty quickly in air, and the longer it is allowed to do so, the worse it gets. 

When you are ready to use the iron, do the following:


knock the solder off into a jar
flash the tip on a sponge (cover the sponge with a paper towel so your sponge stays clean) 
and get to work.
As soon as you're done, recoat the whole working surface with solder.


Tips oxidize dramatically faster when they're too hot, so check your temperature if you can.