I've given a particularly helpful bike shop employee a Starbucks gift card (the shop is across the street from a Starbucks), and I have on occasion given the the mechanics some cash when they have dropped everything to deal with my bike. I don't want to embarrass the guys, so I give the money to the salesperson to pass along with a suggestion that they could send out for lunch (or whatever they want) with the cash.
I figure, why not show my appreciation to these folks? I tip my barber even though I really don't care much how my hair turns out...But they bikes are important.