Here is how simply you can do it: PRESS AND HOLD THE HOME BUTTON - THEN PRESS THE POWER BUTTON. So, press and hold home, then a millisecond later hit the power button. Works flawlessly every time. The image will then saved in your gallery files.
Update:The ICS already released, taking screenshots for Galaxy S2 runs with Ice Cream sandwich is change to Volume Down and Power Buttons..Press and Hold The VOLUME Down and POWER BUTTON simultaneously to take a screenshots.