How to stay knowledgeable with my MOS, without being able to do it?

I went through the same struggles of not being able to do my job. The reality is your job doesn't show itself valuable until your unit is faced with extreme conditions whether it be extreme heat or cold. The best practice is to get deep into your TM. Actually read the book and go through the work packets. Once you've done that do it again. Just because nothing is broken doesn't mean you can't train. As 91c's we are expected to troubleshoot and fix issues within moments because often times the equipment we work on is for life support. The downside is in garrison we cross train excessively to our detriment on other mos'. While it's beneficial for the motor pool that you help them get ahead on services or troubleshoot equipment, they have failed you if you have limited information about your own. I recommend the first step is that you order everything that you need and think you'll need. Then I'd start training.