When booking flights (and probably hotels) use incognito mode. I was booking a flight direct with united.com the other day - found the flight at ~$400 and was entering information. Got sidetracked and the booking session timed out so I started again. New price for the exact same thing: ~$700. Incognito mode (or just clearing cookies as I've heard of this happening but never saw it myself) and it was back to the old price.

So just watch out when booking direct as well.