I developed speed from text MUDs. I'm the type of person who dislikes customizing my setup and prefer to play straight; as a result, I had to consume, digest, and react to pretty large quantities of barely-differentiated text in tenths of a second.

It was good immersion practice.

At some point I just sat down and did some deliberate practice. I'm actually still not as good at typing as I would like to be, I have some glitches like only using my right thumb for the spacebar and sometimes using the wrong Shift key. There are a lot of programs out there that will help you do deliberate practice, it's just a matter of putting in some time.