Back when I had a job flying i tipped if:

They were on-time (they are required to track our flights and arrive when we do)
They are kind
They take my bags and are gentle with them
If they screw up any of these I don't tip...I get paid to little to tip unless they do this.

If they give me a ride to a store/bar/restaurant I usually tip at least a dollar for each leg, usually more.