Allow me to offer a different point of view on increasing your typing speed. When I first learned to type, I actually used Mavis Beacon in school. The only thing that I really remember is that it was easy to cheat on (would you believe that I can type 512wpm with 95% accuracy?). It was helpful for learning, and I was soon typing at 60wpm.
That being said, I don't think typing software is necessary to increase your typing speed. if you type a lot anyway, you can just work practice into your normal stuff. A few years ago, I reached a plateau of around 70wpm. I didn't get any faster for a long time. Then I read somewhere that often when it comes to certain skills like typing speed, once you reach a level you're comfortable with, you plateau and don't get any better because you are no longer actively focusing on improving. The recommendation was to consciously focus on your typing speed by typing faster than you are comfortable with in order to progress past the plateau. I can only speak for myself, but I attribute my subsequent gains (about 30wpm) to this technique. Also, I think it's important to try to move your fingers as little as possible instead of hitting the keys hard and rapidly when trying to type faster.
As for accuracy, I don't know how well this works, but whenever I make a mistake, I delete the whole word (ctrl+backspace) and then retype it. I read somewhere that this ensures that you're not developing the habit of making a mistake on a particular word but have to type it correctly to move on. These two techniques aren't very hard to incorporate into typing I do every day and seem to have worked well for me (especially during my switch to colemak). I think that even people who progress a little slower than some can make gains this way.
Keyboard also has an effect. A mechanical keyboard might help with speed too.
