Try applying a heat protective product before you straighten, e.g. the TreSemmé spray or L'Oréal Thermo stuff (pink bottle, advertised by Beyoncé). If you're in the UK, the very best one is Lee Stafford's straightening mist spray (bright pink bottle).
You could also try applying a little bit of serum like John Frieda's Frizz Ease just to the highlighted parts of your hair.
Good luck!