Make sure you make the salesperson take it out of the tank and let you smell it. Don't buy any live rock that smells like anything but the ocean. If it smells bad, then it is not cured and will make your tank recycle.
Also, look for live rock that isn't just round chunks. The big, round chunks don't allow you to get creative with placement, and they tend to have less of the beneficial surface area that some of the more oddly-shaped, pocked ones.