Trade like you would with real money.
1) Dont put on unrealistic huge size,.
2) Dont endlessly add to losers until they eventually come back your way, you wouldnt sit through a massive loser when real money was on the line.
3) Dont reset your demo balance when you lose money and get pissed off.
4) Stick to your system rules, dont trade like a madman. If you dont have a system down yet, take notes while demo trading and go over every trade in the evening. Hopefully you can then start connecting the dots and get a nice coherent system put together.
For me demo trading became extremely effective after I blew out one account and felt the sting of losing real money. Having that fresh in my mind helped me keep my emotions in the same state demo trading as they would be live, when I got my act together and went live again it was no sweat.