I trust my mechanic 100%. If he says I need anything I make it a point to get it changed out at some point.
But then again, he knows I do my own maintenance and don't need to take my car to him for oil changes, and that's mostly what he does. He only does Mercedes, and has a lift, so when he does my oil change he gives it a thorough once over. I've left my car there overnight for said oil change because he got some emergency "you've got to fix my car now" cases. He calls me, lets me know he has a situation and he knows, i'm going to tell him to take his time and simply tell me when to come and pick it up.
It's also a matter of me having been a mechanic before and knowing how it goes. And me deferring to someone who is obviously an expert in comparison to me. It's also the fact that I have been his customer for a dozen years or so and there is an element of trust in that relationship. I have some customers to whom i am now selling tires for their kids. It's a gratifying feeling to know that their custom is a direct result of my work. But really it's more about having a customer who is willing to listen and engage in the process, rather than come in saying "I need the cheapest tire in X size."