The only thing I can recommend to improve the amount of XP you gain, is to try to have as many assists as you can. Having a weapon like a SMG to land one bullet on everyone or a flamer is really good. Medics/zerkers are the most viable classes atm, so they're easier to play, followed by Commando/support/Firebug, and then, followed by the useless demo. As a level 12 demo, and I can say that you don't want try to get on suicidal unless you're playing with friends that will protect your arse while you're loading that RPG over and over.