I have never had much success with the traditional way the arm bar from the guard is taught. It seems that when I grab the arm, my opponent is instantly aware of the threat and protects himself.
In the following video, I show a way that has given me more success:
In studying the way the arm bar is actually successful in MMA, I observed that this variation accounts for most of the successful arm bar submissions from guard.
Watch Anthony Pettis win the UFC lightweight championship from Benson Henderson with this move.