The best way to train for armwrestling is to spend lots of time on the table practicing all of the angles and techniques needed to improve. Some armwrestlers put massive amounts of time in the gym in order to increase their overall strength. John Brzenk has stated that he doesn't do much gym work at all but puts all his training time into the table.
If you do put time into the gym, some of the better exercises to do are hook pull-ups (instead of your wrist being straight, wrap it around the bar and don't let it straighten out), hammer curls, and concentration curls focusing on both the concentric (hand to shoulder) and eccentric (also known as negative) movements.
Starting Body Position
Starting Arm Positions
Dead Wrist Press