Field monitors should be strong to resist ruff treatment in the field. In addition they should have inputs appropriate to the cameras you use, for example analog component input or digital HD-SDI. Also helpful is built in waveform function for setting exposure as the Panasonic line of monitors have. Size is an issue, as you might want to mount on the camera (8") or else for use as client type monitor (17" or larger). You might consider a larger 23" monitor like eCinema for double duty as field and editing room monitor. Be aware that most LCD's (except the eCinema and Cinetal) have false brightness which makes them not ideal for lighting, exposure reference or color correction.
Monitors come in 1080 and 720 specifications, and again you really want the appropriate "native" display that matches your camera or future camera. It's likely that the monitor will have a longer useful life that your current camera, so try to futureproof this purchase to amortize the expense.