Lineman/judge pre-snap hand signals.

[The Roamin' Umpire]

Page 24 of the most recent (2006-07) NFHS Officials Manual shows a number of recommended intracrew signals. Among these are:

• Holding a closed fist at shoulder level to indicate 11 players on the team that you are counting. At HL or LJ, I hold my arm out to the side, since sometimes it's tough to see when held in front of the body. Also, I always use the arm nearer the defense because...
• Holding the arm on the offensive side straight out to the side to indicate that the player nearest you is OFF the line of scrimmage. A lot of guys call this "punching back", but the book shows an open hand, and that's what I generally use.
• Crossed arms to indicate "two sticks" - that is, there are more than 10 yards needed for a first down, so the offense must pass BOTH stakes to get a first down.
• Hand on cheek to indicate an unbalanced line. I have never seen this used at the high school level, but I like the mechanic. Even better is one I saw a D-I crew using (America East conference): Two fingers held straight out to indicate two players to your side of the center. The other wing responds with a hand on cheek to indicate four on his side; if not, you have a flag.

The easiest way to ensure the correct number of players on the line of scrimmage is if your R and U are good about signalling that they have 11 players on offense; if so, then you can just make sure there are no more than four backs.

As for tips:

1. STAY WIDE. Let the play come to you, if it's going to. As you read that the play is moving away, you can start creeping in, but not too far - if there's a cutback, a reverse, or a really good fake, you don't want to get caught in the middle of the action. Once the play has ended, you can move in to make sure nothing "extra" happens.
2. KEEP YOUR EYES OPEN DURING A DEAD BALL. During roughly 90-95% of the time in a football game, the ball is dead. You still need to officiate. Especially if the last play didn't end near you, keep as much of the field as possible in view. On a play that goes way OOB on your sideline, go with the players to protect them. (Drop a bag on your spot if necessary.)
3. KEEP YOUR SIDELINE CLEAR. Get in this habit NOW. The job is hard enough when conditions are perfect; lacking that 2-yard belt makes it much worse on plays that end at or near your sideline, since ideally you're standing off the field a bit to have the best angle. Be very polite with the coaches, but be firm on this point.
4. YOU WILL SCREW UP. It takes years before all of the things like counting players, checking formations, signalling, etc. become second nature, and they're not even the really important things. Accept now that you will make mistakes, so that when it happens during the game, you can make a mental note of it and then put it out of your mind and deal with the next play. You can reflect on the game later; preferably with the other members of your crew.
5. STAY CALM. As mentioned, you'll screw up. Players will be excited. Fans will be cheering. Coaches will be yelling. And you need to keep your composure and do your job.

There are about a thousand other things to say, but these are five of the biggest points I can think of. (Others will like disagree on point #3, but it really does make things vastly easier for me.)