What helps even more is when I can get something in return for my stuff. *KERCHING*
I tried webuybooks.co.uk not too long ago to get rid of some books and DVDs that I no longer needed. (They also take CDs and games.) It's a web based service where you tell them what you have and they offer you a price for them. If you accept the offer, you can compile an order and send off your items to them. They inspect them, and transfer you the agreed payment. You can read about how it works in detail on their site.
Generally you should have £10 worth of items before you can send an order off, although sometimes they will offer you less money if you can't make it to £10 (i.e. £7 rather than £8). I assume this is due to the cost of processing orders.
I sent them 20 books and 3 DVDs and got £29 in return. It's not going to make you rich, but it's a nice little bit of spare cash. I was happy with my experience.
I found the process pretty simple and easy. The only hard part is you need to box up your items yourself, so you need to find a suitable sized box and then you need to take it to a collect plus location, which for me is difficult. There isn't one near me, or on a bus route, so I usually have to ask a friend with a car to take it for me, but this may not be a problem for most people.
Before I knew it, I'd had my money transferred into my bank account, just as promised.
Overall, I'd really recommend this service - was really great! The only downside is that you get so little for some books, but it's easier than trying to sell each one individually myself on eBay or Amazon where I may not see any returns.