I'm a fairly fast typist (184 WPM on 10fastfingers and 166 WPM average this month on TypeRacer). I'll try to give a few tips for maximizing your typing speed. I'm coming from the perspective of someone who doesn't have a job that requires typing, so the majority of the typing I do is on my own leisure time. And as you'll see, I care maybe a bit too much about my typing speed. Not everyone wants to practice typing every day.
Don't use any tension while typing. As soon as you tense up anything (fingers, wrists, arms, shoulder, etc) you will find yourself choking and your fingers will stop moving as fast as they can. It's easy to assume that a bunch of movement will result in you typing faster, but if you watch a video of any fast typist with the sound off, it actually looks like they're typing fairly slowly. This is because the fastest typists will move their fingers and hands just enough to strike the keys which are needed without extraneous movement.
Know not only where the keys are, but which fingers can be used on which keys. The way you type a word will differ by the words coming before and after it. You should try to avoid awkward jumps and stretches of the fingers, as well as reusing the same finger in a short span of time. This will vary by your hand size and your keyboard. You can practice this by typing at about 75% of your max speed, and thinking about how to maximize efficiency of finger movement.
Slow down during difficult words/letters and speed up on the easy stuff. Typing at a uniform speed will actually result in slower WPM overall.
Know when you are typing well and when you aren't. When you are typing well, you should be able to type with minimal errors, as well as catch yourself immediately as you typo. Even at fast speeds of over 200 WPM, if I am in a good typing state, I can tell immediately when I typo, and backspace the appropriate amount of times without thinking about it. When you are not typing well, your fingers will feel awkward and sluggish, and you will be less aware of the typos you make. If you are not typing well, do not attempt to keep typing through it. This will cause temporary injury and result in your typing speed being slower for several days. Instead, take at least a few hours off, if not an entire day. When you are typing well, expect this to last for a very short amount of time. Personally, I am only about to sustain my maximum speed for about 15-30 minutes before my fingers stop cooperating. Once they stop cooperating, stop typing.
Like anything else, a good practice regiment will give you quick results in typing speed. If you haven't typed yet in the day, take a warmup, typing at a speed slower than your max speed, seeing how close to 100% accuracy you can achieve. Warmup for just a few minutes, then don't do any speed typing for several hours. The next time you try speed typing in the day, you should go as fast as you can on a few texts that you know and recognize, or try some 10fastfingers or typeracer. If you type almost every day in this way (but not too much!), you will find yourself getting great results. As proof of this, I have sped up from an average of 124 WPM to 166 WPM on TypeRacer in just four months, and I am continuing to get faster.
Don't expect that everyday you will get a little faster. Like anything else, success has its ups and downs. You might have a few days or even a week where your typing speed is a little slower. Don't overwork yourself; remember that your muscles are adjusting and that if you keep a good typing regiment you will eventually surpass your old typing speed.
Obviously this isn't for everyone, but playing piano will help immensely with your typing. Even if you're not trying to be a serious pianist, if you have access to a simple, cheap keyboard and some exercises like Hanon (
http://www.amazon.com/Hanon-Virtuoso-Ex ... 0793525446), doing these exercises SLOWLY and with steady fingers every day will give you immediate results in finger dexterity and speed, and these results will transfer over to typing.