This whole discussion reminds me of something which a business guru once told me.
He said every year, he'd take his most prized 20% of customers, aka the 20% who made up 80% of his sales, and would treat them to something really special. Conversely, he would take his bottom 20% of customers, aka the customers who brought in very little income, but took up 80% of his time, and he would dump them.
Now at first we questioned the wisdom of dumping customers who are still profitable, his response was "yes, but think of how much more profitable you'd be, if you're not wasting time with the bottom 20%". It was something which I started doing in my own business, and not only did it make me happier, it also allowed me time to go after the lucrative contracts, since I wasn't wasting time with the customers who spent 3 weeks deciding if they would part with $150.