Be dogged about this phase. The biggest time waste is to leave out sites only to have to go back into the analysis phase later and add them. If you don't have this resource, get a major site you are talking with to do it for you. A subscription to this is quite affordable and there is a lot of information organized by type of site. Check out some resources for media kits online like the ones at SearchZ or 1001 Media Kits.
Nothing replaces some time surfing for sites using search engines and appropriate key words.
Get more than one opinion. Have folks in your department to spend some time on this too. Each person will follow a different track and come up with some unique ideas. Get a site to help you with this. Find out what your competitors are doing and check it out.