I have a Sedu (although I've stopped using it). Make sure to use a heat protectant (like V05 heat protectant spray) before straightening. I always used it at a temperature UNDER three hundred degrees...although it did take a few passes through my hair to straighten it at that temp. I wonder if it's less damaging to use a slightly higher temp and only need to pass through each hair section once? Anyway, my curls did come back afterwards without any problems.
Maybe do a deep conditioning treatment the day after you straighten it.
Hope that helps-