Assuming you want a nice screenshot with the device the following worked for me, from Swany’s answer on this Apple forum thread :
In a terminal, disable shadows on Screenshots with the following command:
defaults write com.apple.screencapture disable-shadow -bool TRUE; killall SystemUIServer
Take a screenshot with Screencapture, either by selecting a region without the Toolbar, or of the whole Simulator window and crop the Toolbar.
You can still add shadow on the image wherever you use it.
The following command reverses the change to the screencapture app :
defaults write com.apple.screencapture disable-shadow -bool FALSE; killall SystemUIServer