Salary is important and in more ways than just having more money. Titles vary at different companies and have different meanings, so salary is the easiest way for a bigger company to place you into a title at that company. Once you're in the company, sadly, title matters since most people at a company don't know what each other make. So if you want to advance your career, you actually have to get raises with every promotion. Don't accept a promotion without a corresponding pay raise -- since promotion means either more work or more responsibility.
Changing jobs is the easiest way to make more money. If you change too often, you'll be pegged as unreliable, so the strategy only works to a point.
Your job should be giving you annual reviews, this is a good time to get a raise and if you don't get one, complain about it. If you don't get annual reviews, asking for one is a nice way of saying you'd like a raise and feedback is actually really nice in either direction (compliments, points of improvement).
Threatening to leave is the worst way to get a raise. But it is important for your employer to understand that you can leave and that you choose to stay -- such loyalty is rare and is something that should be rewarded, with money.
If it comes to it, get an offer in hand from the new company that you are going to go to. Then quit. Don't threaten to quit. And don't bring up money as the reason unless they ask why you decided to leave. Then say "I needed more money, I love working here, but I understand your budget constraints and didn't want to put you in a tough spot." It's business. You're not an asshole for leaving. There's no contract that says you must stay and don't think for an instant that you're so effing awesome that you wouldn't get layed off if they had to do it. It's business. Because it's business and money is now in play (assuming they asked) they might decide to make a counter offer. If they don't, you're taking the new job. If they do, it's because you deserve it and they've decided that the cost of replacing you (explicit cost and opportunity cost) is higher than your raise. Yes, you're replaceable, and despite what others have said, your NP is not fucked. They will survive without you.
By the way, if you're awesome and your boss is awesome, this is never an issue, I've asked for raises casually over beer or whatever, and got them before. A few thousand here, a few thousand there adds up in time. Keep a 5 to 10 year perspective on things.
Lastly, I would point out that salary is not everything. I've made more at other jobs than the one I have now and hated them. It's hard to put a price on happiness.