I just recently figured this out myself, and several people have asked me the method.

So here it is:

First, your phone must be rooted. If your phone is not rooted, download z4root, connect the USB cord to your computer, and choose "permanent root". Simple as that.

After you're rooted, go to the market and search for an application called "Shootme".

When you open "Shootme" you will see a disclaimer, accept it.

Another window will appear that says "Shake mode is now active(blahblahblah)", push hide.

Open the application, or whatever you're trying to screenshot, and shake your phone a few times. You should hear a camera clicking noise.

To open the pictures, download "ES File Explorer" and search for "Shootme". Or on your PC, there is a folder called "Shootme".

Hope this helped. :'D