How can I prevent my soldering tip oxidising while I'm using it?
Some tips which may help:

Do not use abrasive cleaning methods on a new tip. Most tips have a protected layer when new and you don't want to damage that. 

Make sure to use A LOT (Note: capitals AND bold) of solder to tin it. I make a puddle and keep it in there for some time, regularly refreshing the solder or start a new puddle as the solder will 'burn/oxidize' after a while.
In the beginning you will see that the solder does not 'stick', like water on a fat surface it stays away form the tip. Only stop after the solder sticks to the tip like water does to glass. 

I had the very bad habit of cleaning the tip when done with a solder joint. Wrong! The best thing is even to put some more solder on the top when you are done. Make sure to get rid of it before you start a new joint again as the solder will have gone 'bad' (oxidizes). So the procedure is:


Clean tip
Solder using new solder
Put solder on tip
Put iron away for a while.