I've donated as often as I can since I was 17. (Have my regular donor blue card thing now and will go up to bronze soon.) At first I didn't want to because I hate hate needles - but then I thought - if I was sick and needed blood, and the doctor said 'sorry, the population has a collective fear of needles, you're gonna die' I'd wonder why the population didn't just get over it. So I did, and even though I cry every time, I still do it because I know it's important. My mum and one of my best friends would both be dead without blood donors - it's only an hour out of your day, but it really can save someone's life. Plus, free biscuits.
It's easy enough to do, you go to blood.co.uk, search for local sesssions, tell them what time you want and they email you with an appointment. Then you show up, they give you a finger prick test to check your blood's haemoglobin levels (this is quite fun to watch), give you a whole pint of water - you can't be dehydrated - then lie down on a bench and they attach the donor bag to you. You can bring someone with you for moral support if you want, and they don't mind you listening to your iPod or playing on your phone while the donation goes on.
There are some circumstances under which you can't give blood though - like if you're on certain medications or have recently had a tattoo - so it's worth checking before you book an appointment. They have a checker thing on blood.co.uk