If your talking RAWR from the door sticker, then here's not much you can do. However if your truck has a sister truck that can carry more i.e, 2500-3500, then some components may overlap and in theory you can add after market items to beef up the rear. But generally speaking if your truck is a 1/2 ton and you're over the RAWR from the door sticker then there's nothing you can do.
I was in a similar predicament with a 10 F150. I was sitting at 4,000 on the rear with a RAWR of 4050lbs. Being that heavy didn't help the towing any and I traded up to a 2500. Yes I was at the limit, and as far as weights were concerned I was not over but the truck was at max CCC, RAWR and GVW, while still being under GCVWR. Typical 1/2 ton.
What truck do you have?