I find this usually works. First of all, you need time! No rushing. Dry your hair with a hairdryer and then section it off. Use a pair of GHD's but only straighten a small section at a time. Most importantly, don't use your hands. Get a thin toothed comb and run the comb through each section about an inch ahead of the straighteners so that, effectively, you are holding the hair with the comb and not your fingers. make sure you get right to the ends with the comb and the straighteners - the ends are the most important and can make a huge difference to how your hair looks. Use a serum just after you've dried and again after you've straightened. You only need a really tiny amount which you should rub into your hands before application. Too much and your hair will look greasy. Fork out for an expensive serum. You should be looking at paying at least �10 for a small bottle - but it should last you a couple of months.
I'm not a hairdresser, just a tight-fisted skinflint who resents paying anyone for anything! :o)))))))