Click on this link for an excellent reminder from Seth Godin about choosing customers:
http://sethgodin.typepad.com/seths_blog/2009/11/choose-your-customers-choose-your-future.html
Many times salespeople become "POW Prospectors". We have all seen the movie where the hero is made a prisoner of war and stuffed into the bamboo cage and is given slop to eat by his captors. And he thinks "this is slop...but I have not eaten in a long time...and I don't know when I will eat again...so he eats the slop."
Salespeople do the same thing when they decide to work on an opportunity that is either not qualified or not in their sandbox. It sounds like this: "this opportunity is slop...but I have not closed anything in a long time...and I don't know when I will have another shot at a piece of business...so they work on the slop."
Remember - a full pipeline tends to provide courage and clarity...particularly about choosing your prospects and customers.