Most resistances you will get with equipment bonuses (and maybe charms). There is also the survivor's skill Magic Barrier, which will boost all your resistances by 5 each level.
Charms are not removable or replaceable, so unless you will be upgrading the equipment anyway, I would not use small or medium charms. You should be able to find or buy silver / large charms if you have equipment good enough to use for awhile.
Enchanting weapons/armour/items
