Hit the menu soft button, settings, sound and display, then screen timeout. You may have it set too low because you arent using the screen within that timeframe.
(simplified) menu<settings<sound and display< screen timeout
Try going in to settings, then Sound & display, then Brightness. You can select the amount of brightness you like, but don't select automatic brightness. Not sure if this will help, but it seems to not darken so much on mine now.
If you're holding the phone in portrait mode, it's in the upper right corner I believe. Or, you could turn off auto dimming while playing games too. I actually use a widget to control it.