Last year I read the mother of all decluttering books: "The Art of Discarding" by Nagisa Tatsumi. Marie Kondo aknowledges her as a massive influence.
It is pretty hardcore, not fluffy like Konmari, e.g. "If you have it, use it. If you don't use it, don't have it". "When I'm dead, it will all be rubbish".
The one that really made me sit up was her advocacy of just throwing away unread books without even looking at them. Environmental considerations aside, I love books and couldn't bring myself to go that far, but my drawer of books-to-be-read (TDOS: The Drawer of Shame) was getting me down and I felt better after selling or donating all but a couple of these books in January this year. We still have TBR shelf but it's small and I now have a whole dresser drawer to store other stuff like my hairdryer and jewellery 
Freecycle has rarely if ever failed for me. I would also recommend ziffit.com as a great site for selling books, CDs, games etc. They pay better than Music Magpie and they provide a free courier once your trade gets to a certain value.
I think it's important not to get too hung up on how you dispose of things, too. Tatsumi advocates having multiple disposal routes available. E.g. I like to support independent booksellers and have donated bagfuls of decent, saleable books to one in the past. No doubt I'll do it again as I know he struggles to make a living, but this involves train travel to another city and lugging bags of heavy books. Last time round, I thought: "I could do this, but I know I'll just get a "Thank you" and that's it. A coffee would be nice (bookshop is also a café), but I know I won't get anything for free." I did take some books to him in the end, but only what I couldn't sell, and I made over £130 without leaving the house. We are not well off either, and sometimes you have to look after yourself.
Lastly, something Tatsumi said that really works for me was to consider not whether an item's potential has been used, but has its potential been used for me?