I have all the expansion packs, and I have finished all the quests (except kill crawmerax) on both playthroughs of the game, and I am only level 58. I tried going through the last section of the main game and the Knoxx expansion, but killing enemies alone is not giving me much experience. What is the best way to get the experience needed to reach level 69? Are there any other sections of the game with lots of high level enemies, or bosses that I can beat repeatedly?
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.
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).