Online Group Play
Play online in a group. The game hands out more experience points per player in the group. If the group consists of eight players, the game gives eight times the experience points. Playing in a group is also a good way to take on monsters you wouldn't normally be able to fight on your own, especially if your group contains players with a higher level than you.
Chaos Games
Join the multi-player "Chaos" games. For lower level players, the Tristram games help raise levels. Medium characters should play the "Tomb of Tal Rasha" games, while experienced players will gain levels from the "Durance of Hate" or "Baal" games. In the games, players follow a runner into battle against monsters.
Baal
Fight Baal by yourself. If you are in a multi-player group, leave the group just before you reach Baal. Fighting him alone gives you a significantly more experience points. However, only skilled players should do this on their own.
Maintenance
Mind your character's health and defenses. Death can slow down your leveling process, so it's always a good rule of thumb to periodically check your equipment and heal. This is especially important when playing a hardcore character, as they are mortal. Once they die, they stay dead.