For resists you use perfect gems. +20 res +2 max.
Of course prioritize element, because you can't really max all 4 to 90% this way. I think poison might get handled with reduced duration, and then probably push cold and lightning as high as possible Ideally carry one spare item, like gloves with 4 fire max resist gems, so swap it if zone contains dangerous fire monsters.
It always used to be a problem, on a tank barb, who cruises through game with 5% chance to get hit (bear stance, super high defense shield), you often find yourself owned, by white monsters that blast frozen soul type of attack for example. At 75% resist and low hp (pumped str to equip items) it hurts as hell. But with stacked max resist it becomes easy. Remember that even adding 5% max resist to pool of 75%, it reduces elemental damage taken by 20%.