We've been doing this for going on two years now. Off-season--even just by a few weeks--rocks for better pricing. We've gotten a bunch of beautiful places very inexpensively off-season, after school starts, for example...this is in the US; for what we do, we need consistent & reliable Internet access, plus we're traveling with our pet.
VRBO & HomeAway have been pretty solid senders, we've tried a bunch of other options, but I don't think we've ever managed to book via any other site (except for one rental straight through the condo complex in an area not well-served by HomeAway/VRBO). Searching VRBO is a pain in the neck, but the owners tend to be a lot more experienced.
I've been on the road six months so far, and have been using Craigslist, AirBNB, HomeAway, and VRBO.com. (Though these last two nearly never have sane prices.)
You absolutely have to haggle. Every time. Even if you're only staying two days, ask for a discount because you're a quiet computer programmer who knows how to clean up after themself. Other good reasons include last-minute offers and long stays. I've knocked 50% off of ~50 nights so far, the usual discount for even trying anything is 25%, and only once has none of the half-dozen places I've contacted not taken something off the price.
VRBO & HomeAway have been pretty solid senders, we've tried a bunch of other options, but I don't think we've ever managed to book via any other site (except for one rental straight through the condo complex in an area not well-served by HomeAway/VRBO). Searching VRBO is a pain in the neck, but the owners tend to be a lot more experienced.