I think there were three things that were incredibly valuable to me as a brand new umpire.
1. Read the rules. Learn the rules and recite them using the rule book language. You need to know the rules BEFORE you need to know the rules.
2. Read the Umpire Mechanics sections of the books for your organization(s) and follow them LIKE A ROBOT. When the book says to come inside, button hook, drop step, follow a runner, get to a spot, stop and set, give a signal, verbalize (or don't), wait, replay in your head, or whatever else, DO IT until it becomes comfortable. Once the right moves are second nature, you'll start to notice that you're in a good position more often, and you'll understand how to make adjustments when they're necessary.
3. Learn from other umpires. Hang around after you're done with your games and discuss weird things that happened, or things that people argued about, or whatever else seems worth talking about. Ask questions, get opinions, and evaluate them against the mechanics sections and the rule(s). The more you can learn from other people's mistakes, issues, or experiences, the less you have to learn from your own.
If you hang around here, read the discussions, and ask questions when you need to, you'll be a better umpire for it.