The #1 thing that helped my grip was chalk. By far and away chalk. For barbell deadlifts, use a mixed grip. You can also consider a hook grip.
Chalk and a mixed grip has pretty much fixed my grip problems for deads. You can also consider the Captain Crush gripper. www.captainsofcrushgrippers.com/
. I've also heard that pull ups with a towel helps grip. Haven't tried it. Stay away from straps.