Improved iron will. That of course requires iron will as a prerequisite...but hey, you are worried about failing will saves anyway, so the +2 is appreciated. Lightning reflexes and great fortitude also have the same kind of improved feat. Admittedly, you have to decide to use this 'before the results are revealed'...but if you rolled a 2, you can safely guess that you would need a reroll.
Also, you could go half elf. They have an alternate racial trait (dual minded) that gives them an additional +2 to will saves, as well as a racial feat (exile's path) that is basically improved iron will just for enchantments. Combined with the basic save boosting feats, this can make even a fighter have a good will save, let alone a caster like a witch.
Plus, witches have hexes that deal with stuff like this as well. The fortune hex lets you roll twice and take the better result on a variety of rolls for a round (scales up to 3 rounds). That one is particularly useful since it is one of those "everybody gets one per day" things. So you could help the entire party with their rolls. Of course, this is one of those 'use before the roll' things, so it might not be as useful as you'd hope.
Edit for more stuff: Let's see.... a trait from ultimate campaign (kin bond) that allows a reroll, but needs you to be near your 'sibling' (maybe have another party member be your sibling, and you both can take this trait?). It is a bit of a gamble though, since you both become dazed if you fail (you can get a +2 to the save if you are twins/multiple births)
The witch spell nine lives is a late game option that lasts for hours/level and gives you up to 9 uses of a variety of useful stuff as an immediate action.
