The following points are made as a guide to Umpire’s.  The order in which they are made does not necessarily indicate their relative importance.
The Umpire should be alert and pay attention to the game and the players – at all times – whilst the match is in progress.  Do not let your attention wander or engage in conversation or arguments.
An Umpire should move briskly around the table at all times, but should not appear to be unduly hurried.
Make all calls loud enough to be heard by both players and nearby spectators, but don’t shout loudly.
The Umpire should be courteous when talking to players and spectators alike, but firm and decisive with your decisions and calls.
Do not walk around the table holding the rest / spider like a “staff of office”, it has the tendency for umpire’s to lean on them and stand in one spot and also to relax and lose concentration – leave it aside until requested.
Do not let your eyes follow the run of the Cue Ball to the exclusion of all else.  Watch also for fouls around the “cuff area” after the shot has been played.  Some players become very careless after they have struck the Cue Ball.
Look for “danger” spots, i.e. places where the player may foul a ball in bending down over the table.  “Read” the game and try to anticipate what shot will be played next.  Remember whether the player is left or right handed and it usually becomes obvious where you should position yourself to see these types of fouls.
When a player is snookered, try to attain a position so that you can see body fouls etc. but can also step quickly to the object ball [the top players often “lay-up” a ball so gently, that you must be right on top of it to be able to give a correct decision in all cases].
When a player is using a “rest” watch for fouls on the underside of the cue/rest against another ball.
Do not stand in the player’s line of sight.  If anywhere in the player’s “arc of vision” don’t move while the shot is being played.
When racking up the balls prior to the start of a frame, make sure the balls in the triangle are as tightly packed as possible and that the eight-ball is as close to the spot as you can make it.
The question is often asked “where should to Umpire stand to control a game?”  Basically there is no “set place” in this sport.  The player’s intended shot and mode of play in the first instance will dictate where the Umpire should stand.
Other factors which will affect the Umpire’s chose of position are:-
     a]  Balls which the player may foul.
     b] Time available to reach the desired position.
     c] Trying not to stand continuously in front of one section of the audience.
However, as a guide only, keeping points a] to c] in mind:-
     i]   Assuming the player is right handed, one umpire would stand to his right hand side approx 1 metre away from the player.  This will give an umpire full view of any balls under the player’s body.
     ii] The other umpire should be almost diagonally across the table and should be viewing the bridging over balls and the tip of the cue for double hits etc. This way, both umpires and watching both areas where fouls can occur.  With a left handed player, the positions is reversed.
No player is permitted to leave the playing area at anytime during a game without having secured the Umpire’s permission to do so [this is not automatic].
Umpire’s should control onlookers and should not permit them to walk around tables, peering at possible shots.
The prime function of the Umpires is to control the game in all matters of fair and unfair play.
The Umpire is in full control of the game at all times.
Finally – you should always Umpire in the manner in which you would wish your own games to be Umpired.