Touchscreen flow & consistency are probably the 2 most important things. Make sure that buttons that are common to multiple screens are in the same place so the users don't have to hunt around. My POS systems all have the numeric buttons, (I hate popup keypads), plus buttons for Send, Cancel, Print, Pay Screen, Function Screen and a few other functions in the bottom right corner of every screen.
I work exclusively on the Micros 3700, but the concepts are the same for most POS systems.
If you have hard coded item buttons group similar items in columns. Rows and blocks are confusing. ALso, if your screens have color give similar buttons the same color scheme.
Try to keep dynamic lookup buttons and hard coded buttons in the same area between screen. If the user knows that the lookup buttons are in a certain area on each screen it helps to move things along.
Hope that helps,
Pat