One of the bedrock principles of marketing, particularly database-driven marketing analytics, which I won't get into here but is fascinating, is that 20% of your clients make you 80% of your money. This is also known as the Pareto Principle, or the 80/20 Rule. The conclusion it compels is that there are always clients you need to do a better job of servicing, and clients you really ought to get rid of. The devil being in the details, as he is, the question then arises -- which is which?
If you're Best Buy, the clients who make the vast majority of your profits are the ones who regularly buy expensive, big-ticket items, never return anything, ignore sales, always buy the service plan, and never need or want customer service. The ones who make you no money, or even lose you money, are the ones who are just the opposite -- buy cheap stuff with little or no margin, return things, need a lot of hand-holding and rebates, and so on.
Typically, you can do sort of the same analysis with professional services clients, but the rules are different. In particular, there are a lot of "what-ifs" around these clients that are hard to quantify. What kind of business you've done with them in the past doesn't necessarily dictate what kind of business you could potentially do with them in the future.
Over at her blog Golden Practices, Michelle Golden has recently put up a link to an article she's written that discusses how to evaluate clients more objectively, and take some of the mushiness out of thinking about who you really want to court, and who you really can do without. Take a look -- it's a nice example of thinking a little harder about things, with impressive results.