Here's a nice quote from Philip Morgan:
"If you look at your marketing message and say, “wow, there are probably only 10 companies out there that fit that profile”, then you are in the right ballpark in terms of specificity. The reality is that there will be far more than 10 candidates, and those 50 or 100 truly ideal clients will find your marketing extremely compelling. Trust me, they won’t find it compelling if you don’t have the courage to be extremely specific.”