Select your save file: For PC, these are usually in \My Documents\My Games\Borderlands\SaveData\. For the XBox 360, check out these instructions on how to extract and replace your save file using a flash drive. (credit NolaFTW) For PS3, check out this video tutorial.

These actions are taken in the order they appear here on the page. Keep a backup of your save in case this thing messes it up.

Playthrough to fix:

Complete playthrough (and enable next playthrough)

Complete side quests

Reset playthroughs

Reset side quests

Reset specific quests

Advanced, Experimental, and Dangerous

Backpack space and fixing overleveling

or, set backpack space to slots (cannot exceed max legit value)

Lower level by: levels under current quest