Pressure Sensitivity in Shi-Painter
For activating pressure sensitivity in an oekaki applet (most likely Shi-Painter), you’ll need a proper drawing tablet and the small JTablet software by Cellosoft. Note that it’s only available for Windows, and may not work with all tablets — Wacom is the industry standard when it comes to tablets and they seem to work the best; products by brands like Aiptek or Trust aren’t all that reliable.
Start a new drawing in your favourite oekaki site’s (like, say, kupoli.net) Shi-Painter app, and find the small “P” button in the right-hand corner of the drawing area:
opens a window like this: 
The two sliders, when turned On, control stroke transparency (alpha) and size. The upper slider is for minimum value and the lower one is for maximum. So if you want a stroke that goes from tiny and transparent to large and opaque, leave the upper sliders emply and maximize the lower ones, and so on. Experiment until you find the combination that works best for you.
It’s worth noting that updating Java usually makes the pressure sensitivity stop working, so after every Java update you’ll probably have to reinstall the JTablet program.