My hair's basically long and straight anyway, but in damp weather, it tends to curl slightly towards the bottom, so I use Jogn Frieda's Frizz-Ease straightening spray. Works beautifully, and leaves your hair soft and shiny.
Use Tresemm�'s Heat Protection Spray (black bottle, purple text). I usually hate their stuff, but I've used this for ages and it's great. It keeps your hair really healthy looking and soft. Two of my mates are converts to it, too.