Reverse washing/conditioning. I wash my hair every day (Which I realise is not good but I've done it for years and not about to stop) but day 2 it would be greasy so would either tie it back or just wash it again.
I then heard about reverse washing, which is basically wetting your hair, applying conditioner, leave it for a minute or so, loosely wash out with water, and then shampoo your hair. Don't re-condition.
Some people find it hard to comb through afterwards, but personally I don't have this problem. Then blow dry as normal. My hair is much softer afterwards, and doesn't get as greasy and I can get 2 days out of it, and ebven wear it down on day 2 (previously unheard of!). It feels weird at first, but you soon get used to doing it the 'wrong way around'.
There are 'specialist' shampoo and conditioners for this process (Tresemme do one), and whilst they're good, I do it with any bog standard shampoo and conditioner.
Another thing I have discovered which is brilliant is Percy & Reeds 'Smoothed, Sealed & Sensational Volumising No Oil, Oil' which you can get online. M&S sell it too (I'm going to stock up on a 20% off day). I got it as a sample and its really great, doesn't weigh your hair down at all. My hair has taken on a new lease of life!