I think Complete Warrior also had a feat to increase DR. It is inferior to the Savage Species feat, though, if that book is allowed for you (isn't it technically 3.0? Not to mention disgustingly broken...).
In Magic Item Compendium, the Armor of the Watchful Master (I think that's the name) is a set of items which improve DR. It stacks with permanent DR, and seems explicitly made for a barbarian to use.
In any case, you're getting the DR from magic items, or class/race? Because I don't think there are ANY avenues to help you if the former.
Not actually DR, but there are armor crystals in the MIC as well that let you ignore 1, 2, or 3 damage from an attack (respectively, for the least, lesser, and greater versions) 10 times a day.
Almost certainly unhelpful, but if your character happened to be a Goliath Barbarian, they have a racial sub. level to replace the barbarian's DR X/-- with DR 2X/adamantine. in other words, you get double the bonus, but it can be overcome by adamantine.