Know that you deserve it. I keep an up to date LinkedIn profile, and when recruiters contact me I ask them if the position they're hiring for will pay more than a certain figure (my benchmark is usually 50% more than my current annual salary). Your benchmark value may be higher or lower, but either way it should help you gauge market conditions.

Also when you're taking a broader market into consideration, be sure to consider quality of life. Yes I may get more money somewhere, but it might be a miserable product/team/culture which reduces the value of that position. Or I might have a shorter commute, fewer hours or more vacation, which would add to the value of the other position.

I search for other, comparable positions, and see what I _could_ get if I jumped ship. At my previous job, I asked for a raise when I realized I was being paid significantly below market rates. They gave me a decent bump, but not enough for me to stay at the company when I got another (higher) offer.