TopShelf2010:Train yourself to select the putter scale you desire before pulling the meter back. Once it becomes habit, it is likely to not happen again in the future.
As Don noted, apply a methodical approach to every shot, putt or not, that includes checking the club, shot type, wind direction, putter scale, et al, before starting the shot. Sometimes I realize I moved the aimer in the wrong direction for wind, or some other silly blunder. Being a little off on the ding or percentage of power is not usually a disaster but using the wrong putter scale, wind direction or club can cause big problems!
We all go through this, and even then, we still screw up sometimes!