How do you level up quickly on Borderlands

In the first mission (Fresh Off The Bus) After you kill all enemies, before you turn in your mission you should level up from them. Then quit the game and reload it. All the badguys should return, giving you exp each time you restart.

When you get the Circle of Slaughter mission, go there and fight all badguys until you get to round 3. On round 3, fight all enemies until there's one left. Don't kill it or this technique will be ruined. restart your game and you should be able to restart round 3, giving you max exp.