I find classics have a really high awesome-to-meh ratio (as you'd expect, really), so if I really want a GOOD book I head for something I've heard of but haven't gotten around to reading yet. It's harder to run out of classics than you'd think.
Other than that, I just browse the library and pick up what looks interesting. Often things I've heard friends recommend - occasionally on MDC, although I recently read a few clunkers recommended here, so you never know! - or books that were made into movies that showed at the arthouse cinema where I used to work. Or even just "I've heard of that author but I don't know where", or - as a recent rash development - books that have catchy titles or interesting synopses.
I don't know that many contemporary authors, so just diving in was an attempt to get over my snobbish no-modern-writers-are-really-great thing. I've found plenty of unspectacular books that way, but it doesn't matter hugely because I have a lot of spare time at the moment (well, morning sicknessy lying-down-is-my-only-option time, at least) and I read really fast. So if I get through one lame Helen Fielding knockoff chick lit book and a pretty interesting non-fiction book one day, and then a fairly well-written Angela's Ashes-type Irish childhood book the next day, and then finally come across a book of really quirky short stories... well, it's better than watching the telly.
Anyway, looking for specific books at my library is frustrating. Either they don't have 'em at all, or they're out. It's less pesky just to wander the aisles going "Oh yeah, David Sedaris, someone on MDC recommended him... hey, wasn't Anne Perry the girl from Heavenly Creatures?... oh yeah, Evelyn Waugh, I forgot about him... hmm, I wonder if I could read a Stephen King these days without freaking out..." until the pram's full.