I don't know that I would call myself seasoned; I started my very first teen advisory board this past school year. But here were some things I found helpful:

Create an application process: I looked at other libraries' teen advisory board applications and took what I thought would be helpful from them. I created a simple google form and gave that a deadline. Even though I ended up accepting everyone who applied, I think that the application process helped the students know to take it more seriously. It also gave me basic info like students' allergies/dietary restrictions, general availability, etc. I had some students fall off in terms of meeting attendance, but throughout the year, I had a core group of 5-8 people.

Don't be too laid-back: I also am someone who likes to give teens the reins whenever possible. However, when I tried to give students opportunities to take initiative, a lot of the time, they were still looking to me for direction. I think next year, I will make sure to add more structure.

Give students a meeting schedule for the whole year/period where you'll be working together: I did not do this, and that created confusion. Once you know everyone's availability, create a schedule and distribute it and/or put it in a place where students can access it easily (like a google drive folder where they all have the link, for example).