For me, since my bench is so shitty, I've been working on getting everything stronger. Tons of triceps, dumbbell work, lats, upper back. Shirt work has seemed to help my lockout a bunch so now I'm working on hopefully bringing up the lower portion with heavy dumbbell pressing.
As far as 1 accessory, I'd say get bigger. My theory is, I'm not going to bench 400 at 200 BW with 16" arms (I'm about 6' tall), so while I'm doing everything I can to get stronger, I'm also working on getting bigger.