As GOULI asks, what type of tasks do you usually perform? My method when I don't have much of an idea on exactly what I want is to grab a price list from a shop with a large range (just as a guide so doesn't have to be local) and pick something mid-priced and look at the features you're getting. If you don't understand the features do a bit of research and decide if they're significant to you, if they're something you just want or if you couldn't care less.
It takes a bit of time but by looking at different price points you'll build up a picture of what you want in a monitor.
Keeping an eye on eBay is a good suggestion once you know what features you want.