I hear people talk a lot about burnout. I actually just talked to our staff about the subject this week at our staff meeting and here's the deal: most of what we can burnout isn't burnout.
When I first became a Christian, I served several days a week. Looking back, I was probably serving 20 hours a week, plus a full time job and being a full time student. Why didn't I burn out? Simply put, because Jesus saved me and I counted it a privilege to serve Him.
We need to be careful that we don't put the burnout idea into the minds of volunteers so they think if they serve 3 weeks in a row they're going to fry.
Guess what? After serving three weeks in a row, you won't burn out. In fact, you won't even smell like smoke :)
Do we need to be cognizant that burnout happens? Yes. I'd recommend Wayne Cordeiro's book "Leading on Empty" as a great text.
But trust me, people serving as a greeter isn't going to burn them out. Especially when the joy of the Lord is your strength.