A scattering of high level elemental gems on armor to boost your resistances. See next line.
Skulls are always good. Consider putting one really buff one into your main weapon to enhance health steal, but definitely put all the others you have about your armor/bling, because they boost Def. Boosting Def increases both your armor damage absorbtion and each of your resistances by a percentage equal to your current Def value. Does not buff missle reflection, damage reflection, or Block, however.
The combination of the above two effects can have great impact on your overall elemental resistance.
Pure Embers or Core embers are both good candidates for the other slot on your main weapon. Or just consider putting the highest damage producer gem you have into the second weapon slot. Pure Embers are the big physical damage producers. Core Embers in weapons give you
Never put Chaos Gems, Fish Scales, or sea creature Teeth into weapon slots. Their effects, even the damage effects like +3 Critical Strike Chance, are not contingent on what type of slot they are in, and the very few weapons slots that you have are precious for gems whose enhancements are Item type specific.
Don't worry too much about enhancing your stats other than Def. Your main stat is almost certainly getting plenty of increase from your regular promotions, and the two other stats are generally much less important to playing a particular character type.
Socketing a Mana Steal gem (?eyeball?) could be more important than Health Steal for a ranged character, because you are probably burning through mana-from-range more quickly than health. On the other hand, a couple of layers of Adv. Spellcasting could take care of that as well.