Certain shampoos are stronger then others and so strip the hair more. I can't quite remember which shampoos are the worst though I'm afraid.
If it is windy your hair will get greasy a lot faster as well, could be a conbination. It is a good idea to give your hair a break regulary, you can get really nice headbands that cover a lot of your hair or hats. Perhaps try and play around with waxes etc to find a style that doesn't need straightening.
If you hair is in a really bad condition go to a salon and ask them to look at it, and don't forget to get regular trims..