Is there are some mental techniques through which one can increase his or her speed
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Practice. There is no silver bullet. Just practice. And have a clear picture in your mind of what you want to write.
After many many years, I have brought my typing speed up to about 100 WPM on a standard QWERTY keyboard. What I have found, is that at that speed you have to stop thinking in terms of typing specific characters but in think and type in terms of words and to push yourself a bit.
Now I am in the mode where I work from just a summary or an outline and can generate whole paragraphs. But again you have to think in words or groups of words, otherwise you'll not be fast enough.
I also pushed myself because I had lots of interactive chat sessions via a one line typing interface per person. You got faster or the other person would just fall asleep waiting for the response.
I've started a program to teach myself the DVORAK keyboard system, but it is slow going. I'm not getting much over 10 Words per minute and a lot of concentration is needed to prevent from falling back into the QWERTY mode.