Work with clients who are happy to be happy. Fire those that aren't.
Customers come in all shapes and sizes.
Some have fat accounts. Others barely have budgets to match their dreams.
Some can see your vision right after you're done with the last sentence of your pitch. Others will question your every idea until the cows go home. Give in, and you and your idea are screwed. Fight for the campaign that you know is a winning one, succeed, and you'll find yourself a loyalist.
After 11 years in business I have encountered and served them all. Quite happily. In over a decade I've fired 3 clients because of their energy drain.
I figured out in year 5 after a particularly clueless client berated one of my employees without cause, that I would obsess only with delighting customers who were happy to be happy. Does that sound like tautology? It's actually not. It has become the raison d'être for our successful PR practice.
Work with clients who are delighted to be delighted. Hire wonderful, talented people who know how to serve them.
Happy customers will spread the word. Happy employees will attract them. To me that's win/win
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In the end though I judge the importance of a client not because of spend, but by how challenging their work is (I love the hard projects) how much they spread the word about my firm's work, and how much trust there is between us.
When there is trust and respect, a client is so much easier to please.