According to this helpful link, you can:
1. Boil SPs whole until tender, peel and slice/cube them and, when cool, freeze.
2. Bake SPs whole, cool, then wrap in foil, freeze (& reheat whole in oven up to a year later).
3. Mash the SPs (with some lemon to prevent browning), freeze (& use in baking/casseroles).
(ps) I also read that using baked/cubed/frozen SPs in smoothies is good, but have yet to try it!