Allen,
I would agree with Andy about not recognizing years of service but rather actions. I'd cringe if people started comparing their lengths of service with each other at bike farm. "I've been here 5 years! Longer than anyone else in this room." It's the kind of quantification that can lead to weird power dynamics and hierarchy. I'd much rather someone remember what I've added to the org over the years, then my length of service.
At the same time, I have thought about the same issue of how to best retain volunteers and energize around making Bike Farm the best it can be. Over the years I've found that there are four great motivators which garner dedication in a volunteer:
They are actively helping people. This one is the most obvious and it needs to be there. People need to feel that what they are doing is good for the world and that they are making a difference.
They are getting something special and real out of this. We have some shop perks which include ordering privileges, monthly $20 credit for used parts, and free stand time for volunteers who have hung out for at least 2 months and hold down a shift.
Volunteering is helping them in their career/how they make rent. This one is slippery since the most common expression of this motivator is a volunteer getting a job as a bike mechanic. Being a dedicated volunteer looks great on a resume, especially if you have a fancy title like "Shop Coordinator". But often when folks get that desired job, they have to take a break from volunteering to focus on it. Still, this outcome is the best we could hope for, and often those volunteers come back better trained after a few years. I always let volunteers know that they can put me on their resume as a reference. I have written many letters of recommendation over the years. That's a real perk for some people.
This is a social outlet for them. This is by far the greatest motivator for volunteering. Making friends and building a community strengthen dedication. It's what hooked me back in the day.
With these things in mind, I try to think of every action and see if it fulfills one of these drivers. Volunteer parties are great for the social reason. Soliciting awesome donations from big name bike shops or manufactures is great for the "getting something real" motivator. And when talking to people, I try to understand which of these factors is the greatest motivator for them and I try to help them achieve it.
So ask people what they are here for.
I could talk way more about this topic. There's a whole other dynamic of volunteering in that you only get new dedicated volunteers when there's a hole to be filled. But I'm curious to hear what others have tried.