This is one thing I'm still trying to work out. Writing fast without thought, without pause just keep writing. I'm starting off slow with this and build it up. The idea is to get your subconscious in the writing frame of mind so that words just flow. I've been practicing by being given a few characters and a location. I then cover my computer screen so I can't go back and check what I am writing. The aim is to just write as muh as you can. To challenge you further set a time limit like 10mins. I started off this way and got less than 50 words. That's because I was thinking not writing. I have since then improved to 100 words in 10mins sometimes I get more. It's a way to reprogram your mind on how to write and do it fast. This also helps get through blocks and puts your words on paper quickly.
Example of a Challenge if you'd like one.
3 Characters and a location
Student male or female your choice
Executive Business man
Cafe by the Beach
Give it a go.