To excel in powerlifting you need to be analytical. Assistance work will best serve you if most of it addresses a motor pattern, muscle group, or conditioning problem that is holding you back. To that end, assistance work can and should vary radically between lifters.
Most powerlifters will have a few things in common though. Lots of posterior chain work (box squat, GM, reverse hyper, etc.) and emphasis on glutes, hams, and lower back; extra triceps work and lockout training, conditioning that improves work capacity specific to lifting.
Right now, I am focusing on hypertrophy (to move up a weight class). Other specific needs I have are even bench lockouts (unilateral triceps work), strength in the mid back and upper spinal erectors to aid pulling back in the DL, and rehabilitating my shoulders and pecs from several injuries last year and earlier this year. So I am using fairly high volume and working in things like 4 board presses, rack lockouts, dumbell triceps extensions, heavy dumbell rows and pendlay rows, bent back safety bar goodmornings, and mobility work for the shoulder girdle. I for one do believe in lots of ab work so I do heavy (5-8 reps)training during every squat and deadlift workout, focusing on weighted situps or standing cable abs with some torso rotation work and plank holds as well. My conditioning is typically sprinting or a variation of sled pulling for high sets and short rest periods.
Hope that's what you're looking for. Good luck starting your PL training!