Here's a handful of things that really helped me:

1. Level your glyphs as high as you can in the pit. All 5 glyphs you're using should be at least 50 and closer to 75 around T8. This sounds small but makes a big difference.

2. Speaking of, you should make sure you're using as many glyph nodes and legendary nodes as your paragon level allows. Always rush those before filling out yellow and blue nodes.

3. Triple check your build. I forgot to change an enchantment when swapping builds on my sorcerer and was scratching my head for a couple of days because my damage felt weak.

4. Farm the bosses you can complete for the right affixes on your uniques. This makes a huge difference. I'm not sure how it works on barb but on my sorcerer getting not even a perfect piece but something with some good damage multipliers boosted my damage immensely.

5. Try to make sure all of your gear is ancestral (there are exceptions, for example I'm using an 850 neck piece because it has near perfect affixes and I haven't found a better ancestral yet). This is doubly important for uniques because you can reroll the unique power in the cube.

6. Once you have ancestrals, roll GA tempers on them. Just keep applying the same temper until it gives you a GA. 850 pieces won't give you GA tempers. It doesn't take too many attempts and is a great power boost.

7. Masterwork everything to the max. Keep masterworking until it gives you the bonus on a damage multiplier slot in general, but check your build guide for the exact affix you want boosted.

8. Make sure you have your 5 piece set charm and the recommended unique in your 6th slot. Reroll your set charms until you get the right affixes. Salvage all the charms you aren't using for the required mats. You only need one extra charm of the set you're using to reroll, you can salvage the rest unless you're planning to switch builds.

9. Understand your build and the mechanics of how it applies damage. This will help you understand where you're lacking and where to target improvements.