I have done this for 21 years, 10 high school, and 11 Valley League Seasons. Some of my advice will overlap others as you have gotten some good stuff.
1. Always be 100% sure the mic is off when talking in the press box.
2. Get the lineups as early as possible. I write out their number, name, position, college, and hometown.Large Print.
3. Get hooked on phonics-for any name you are not 100% sure of the pronunciation, go to the dugouts and ask. Then write it down phonetically.
4. As to nicknames, be careful. I've had college guys set me up with goofy names.
5. Don't assume a substitution isn't made unless you are told. Double check the batter's number as he walks to the plate as all substitutions are not timely reported.
6. Bring a bottle of water.
7. Go to the bathroom right before you start-the most important rule!
8. If you play music, never play a song that you haven't listened to first. There are many good songs that have inappropriate words, but you can often find "clean" versions on line. Make sure you acquire all music via legal downloads. Normally you need a license from ASCAP to play music at games with paid attendance. Normally they warn first and contact you to get a license.
9. Never play the Three Stooges Theme. I did that once...not realizing it's the "Three Blind Mice" song. I apologized to the umps afterward. Fortunately I knew them all previously and they laughed. Something like this can get you tossed.
10. This is probably more for college level, but if you do sound effects/music clips, never ever be malicious. There are lots of cute clips. Cute is okay, but when in doubt, don't play it. When a coach talks forever on the mound, I have a Daffy Duck that says "Could we please make up our mind?" Most coaches just grin.