I have no justification for acquiring more in the next twenty years or so, but do anyway - I have lots of library books out, I buy books via Amazon, Book People and Abe Books, and in the station WH Smith on special offer. I try to support my local indie bookshop by at least buying something there when I get the chance. I love charity shops and have a particularly great one, with lots of good books and very low prices, near me at the moment, I rarely leave there with less than 8 books, though that does include books for the kids now and in the future too.
I use Read It Swap It and Bookmooch too.
I also get books in exchange for reviews -
I review for www.thebookbag.co.uk - 48 books since August 2009, inc about 43 this year
www.Librarything.com has an early reviewers
scheme.
Waterstones gives away books, I've only managed to get one so far though.
If you use twitter, W H Smith will send you books for review if you send them your details, but they send very random things - everyone else gets books I really want to read, and I got a preposterous chicklit book and a rather better thriller about a mafia type family, which I'm still reading but probably wouldn't have picked up.