Don't let prolonged mauls develop. If someone's held and clearly not going to ground, call "use it " and then quickly blow (in which case they lose it). 7's is not about wrestling for the ball.
Many tourneys will allow the conversion kick back infield (so from the In-Goal or behind, dropping over the post and infield) rather than hoping some kind soul will go and collect the ball. If this isn't the case, let 'em know before the start, and suggest they position a player behind the posts to collect the ball after the kick.
As to running lines, you'll often find yourself running ahead of not only the ball, but also the defensive line. This is not as bad as it sounds, and can be a very effective position. Try to be in the 22 for each score.
Time very rarely goes off in 7's. The watch is running, so let both teams know this if one or other seems to be dawdling. If the losing side, it's tempting to think it's just their lookout, but don't forget the winning side may need to rack up points to progress. If the winning side is wasting time, consider FK options.
Remember that a ball can boiunce inside the 10m at a KO, then go longer. I saw a L.7 ping every time the first bounce was not 10m, even when the ball subsequently cleared the 10m line.
If you are asthmatic, DON'T FORGET YOUR INHALER!
There will probably be a 1st aid tent and/or ambulance. If the pitches are well spread out, get the mobile phone number of the ambulance driver or first aider, so you don't have to send a runner to attract his attention while a player either bleeds to death or lapses into unconsciousness. Then remember to take your phone to the pitchside.
have fun