I'm in line for a promotion to manager, and I really don't want it. However, my company's culture is all about getting ahead and moving up. Everyone is very competitive and eager to rise in the ranks. Sure, I'd make more money, but I'll have more headaches and frankly, I don't think I'm management material. I like the job I do now. Could I buck tradition and turn down the promotion?
Before you can decide what to do, try this exercise. Take a piece of paper for each alternative you might face (i.e., stay where you are, take the promotion, quit and change jobs) and write down the worst that could happen in each route. You'll probably find that to do this exercise you'll need to gather some more information.
When you're done, consider the least dangerous avenue. If it turns out to be staying where you are, you can gently explain how much you enjoy your job now and that your employer might be able to enrich your current job with added responsibilities that match your needs. Your manager can think of it as a horizontal promotion (up isn't the only way). He or she would know that you're willing to take on additional tasks, yet you wouldn't give up the job that you love.
If it turns out to be the promotion, negotiate a potential fall-back position if it doesn't work out. This will allow you to return to your former job without negative consequences. If you take the promotion, you might find out that you like the new job and can perform better than you'd anticipated.
If it turns out to be a new job at a different company, begin an active job search now. Just be sure to take a position where there's no built-in promotion for the foreseeable future.