Some answers

As the pressure or speed of contact and separation increases, the amount of the static charge buildup (voltage level) increases. 

Source: 

http://www.tekpak.com/pdf/eng_article3.pdf

So, from this source, I gathered the necessary information and was able to provide answers for some of the earlier questions. 

- Pressure does affect the amount of static electricity generated

- Since more pressure helps increasse the amount of static charge buildup, I think that I should apply more pressure when rubbing

However, this would lead to another problem. How do I measure the amount of pressure that I use? Sometimes, i may unconsciously use more pressure. 

So, in the end, I decided that I should just use my normal strength. Since it is difficult to measure the amount of pressure applied, then I should just change the factor.

Instead of increasing the pressure, I could increase the speed.

I can keep my speed constant by using a metronome. By following the constant sounds, I will be able to tell whether I'm too fast or too slow.