Grip holds work wonders on your deads. Also rack pulls since you don't have to control the weight through the whole range of motion.
Here's my philosophy: "If you want to bench more weight you banch more weight, if you want to squat more weight you squat more weight, so if you want to be able to hole more weight you...." I tell people this at the gym all the time when they ask for tricks/an easy way to build up their grip.
There are no tricks to getting stronger holding grip. Grippers don't help all that much because crushing grip and holding grip are two different facets of strength. I worked up to being able to rep 770 on rack pulls without straps, after 860 I put the straps on but hopefully this next heavy phase will get me up much higher.
Cheers,
Scotsman