As others have said, know you value to the company. But, there are two parts to that. First (as already stated), know what a competitive salary is for your position and experience. Second, be able to back up your request. Have documentation of goals, successes, and anything else you've done to move the company towards its goals. The latter is worth a lot more than an article or job posting telling you what you're worth (and generates a bit less animosity than waving an offer letter in their face).