If your boss/client is so pigheaded that they won't see your well-reasoned point, quit. Get another job.
This advice isn't very realistic or helpful. It's easier said than done especially given the current market conditions. Besides, if everyone followed it, we'd all be working for the same two or three companies... and tilly would probably still walk among us.
-sauoq "My two cents aren't worth a dime.";