I think that part of the problem is that we do all of the training at the beginning of the spring season, then turn umpires loose with absolutely no follow-up. I've never, ever been officially evaluated except at tourneys, so without any constructive feedback, I only have my own motivation pushing me. The more constructive feedback you give a crew, the more likely you are to keep them motivated (or at least weed out the bad apples).
Communication is also key. If there's no dialogue with the crew, there's nothing keeping them focused. Use email mailing lists to send out newsletters to the crew. Give them "plays of the week" with a question about a play, then your message to the crew, then the answer to the question at the end of the email.
Here in NC, we have the spring season, a 6-8 week break, then the fall season. Both seasons have an equal number of teams, so there's plenty of ball to go around. However, we don't have any meetings during that break. I personally think that's the perfect time to regroup, hold a brief clinic, then send the crew back out.
The more attention you devote to your crew, the more involved they will become. While it's not their full-time job to be umpires, if you look after your crew as a full-time supervisor would to his/her own employees, you'll see results. If week after week, our bosses sent us out to get fried, we'd have less incentive to stay. If you show that you will invest in them, back them up and give them the tools they need, they'll be more likely to stick around.
As for the uniforms, that's a tough one. You could set up some sort of "loan" for them and allow them to deduct it out of their pay, but that's also a risk for you if they decide to bail after 5 assignments. Maybe go 50/50? I don't know. Probably a terrible idea, but I'll gladly open the floor to anyone who has a better one!
