Reach level 69 in Borderlands

Solution 1:

You can go back and fight Motorhead. He respawns every 15 minutes and he gives out experience each time you fight him.

Also, fighting SkyScraper at Level 57 nets 23,000XP if you kill him on the ground. Aim for his rear end (for critical hits).

Also (and I can't believe I just remembered this), the higher level challenges unlock about 20,000XP a piece.

After you beat General Knoxx, the enemies are 'leveled up' to levels 55-69. This allows you to gain experience by fighting them again.

Solution 2:

While George Stocker's answer is a good one, I personally obtained level 61 by playing with others. You won't get as much experience (depending upon which games you join), but the number of players in a game does increase the enemies skills (and drops, if I remember correctly).