The button combo thing doesn't work the same for all versions of Android, and on some phones it won't work at all.  The beauty of fragmentation, I suppose.  Back and power buttons work for some Android 2.2 phones, and back and home does on others.  Power and either back or home works for some 2.3 devices.  For 3.0, I don't know of any combo that works.  It is easier on 4.0 devices, which use volume up and the power button.  

There is an app that works on most non-rooted Android devices, but it cost $5.

https://play.google.com/store/apps/details?id=com.edwardkim.android.scre....

If you have rooted your phone, there are free apps on Google play that work great.  I use aScreenshot, which lets you take a screenshot by shaking the device or pressing whatever button I assign to trigger it.  

https://play.google.com/store/apps/details?id=com.longxk.ascreenshot&fea.....