A while ago this has been another thread. It seemed like there were different methods in different versions. The method above mentioned is the right one for my Ace, although I have found it a little easier to use it in a slightly different way because pressing both home and power button at exactly the same time wasn't really my thing: first I press the home button (a popup "Task manager" appears). Keep pressing the home button, but push the popup away by pressing the back button, then add pressing "Power", then release the power button again. Mind you: during all of this you still keep pressing the home button! After releasing the power button the screen shot will be made. You can find it within a newly created map on your sd-card called "Screencapture".