The single biggest tool a mentor has is communication. Build personal relationships with your students. Get to know them personally, find out what they're struggling with, what their passions and aspirations are. Build rapport and build mutual respect. It is very hard to be an effective mentor if you don't have the respect of your students, and it's very hard to earn the respect of students if you don't respect them. It sounds obvious but it's not easy. They will have good ideas and bad ideas, and so will you.
As you gain mentor experience through multiple seasons, the age difference will become more pronounced. Never forget that they're still high school students. They will still goof off (they need to, and so do you!), and they will still make mistakes, some of which will seem stupid and obvious to you. You MUST allow students to make their own mistakes. I have found one of the most successful mentoring "tools" is to suggest a students try something in a particular way. If they choose to follow your suggestion, great. If they choose an alternate method (say a different machining technique) that's OK too. If they decide to switch to your method and it works better, they will respect your idea more without you having forced it on them. You never want a student to resent doing something because they weren't allowed to do it their way (unless their way is unsafe, in which case don't let them). Understand that you will make mistakes too. Apologize for them if you realize it.
I try to have discussions with other mentors while students are around, if the subject is relevant and appropriate. If I'm working on a design problem with a student, I will often call over another mentor to bounce an idea off of. The student may or may not have enough experience to contribute to that discussion/decision, but they will hear, absorb and retain it. They will also appreciate that you don't have the magic right answers, but that the process of engineering involves many people with experience providing opinions and double checking each other.
Three final thoughts: Firstly, mentors can have mentors too. I have many people in FIRST whom I look up to for their skills as a mentor, and have learned from.
Secondly, you can always learn from your students. They will blow you away with creativity, enthusiasm and often maturity beyond their years. We can always keep learning new things, it doesn't matter from whom.
Last but not least, my personal golden rule of mentoring:
Be passionate about what you do, and let it show. Passion for FIRST is infectious. Students will work much harder on a problem and care more about it if they feel like it is a priority to you and the team. It's OK to get into heated discussions as long as it remains respectful. It's OK to be disappointed with a poor performance, or be frustrated by something not working right. It's ok to lose your voice screaming because you won a tough match that looked hopeless, and have your voice still be gone 3 days after Championships
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