Sign up for an Airmiles scheme
Avios and Virgin are the main ones that spring to mind. If you plan to travel more with Virgin then it's best to just sign up with Virgin miles and reap the benefits of being a member, if you prefer BA or other airlines it's best to register with Avios.
Now I'm mostly going to talk about Virgin as I have first hand knowledge of this, but I've included the link to Avios so you can have a nose and see what it's all about.
If you sign up with Virgin you have the extra benefit of saving an extra 7% off your holiday with them. Now 7% doesn't sound like a lot but when you're spending hundreds or even grands 7% knocks off a good piece of change, it's a noticeable amount. Also as you move up the membership ladder (as they start you on Red, then you move to Silver then Gold) the more you save and obviously the more benefits you get.
So even if you haven't taken your flight yet you can still sign up then once you book you can give the agent your frequent flyer number and get your little discount. Well... actually make sure you ask for the discount as they may not automatically apply it...
So that's 2 discounts there already, you're well on your way :)