I enjoy watching games, and I enjoy watching other umpires work, especially those that are better than I am.
Some guidelines:
1. Make absolutely no comments about calls, mechanics, umpire performance or ability.
2. Be out of uniform to the extent possible. If at a tournament, at least change shirts and don't wear your official hat.
3. If anyone (fan, coach, player) recognizes you and tries to get you involved in a call or discussion, state clearly and (if the umpire on the field knows who you are) loudly enough for the umpire to hear "Not my game, coach (or whoever)."
I will even go so far as to move from one "side" to the other in the stands if people know I am an umpire so people won't associate me with one of the teams.
ALL of this goes completely out the window, BTW, when my DD is playing! (JK - maybe)