I usually tip 20%. So, if the total fare is going to be $120, the total tip is going to be about $24...so I split it up and tip $12 each way. I refuse to tip for the whole trip on that first leg to the resort. If I get a different driver, and that has happened before, then I have to tip again. I doubt that the first driver is going to split his tip with the second driver!!!
But...if I don't receive good service, then the tip is reduced. I want that driver waiting for me, I want my bags to be taken care of, I want a clean, pleasant vehicle. I want the driver to be pleasant and cheerful. I want my bags taken out of the car when I get to my resort. For my return ride, I want the driver there, ontime, waiting for me in a conspicuous spot, I want my bags taken care of..pretty much the same as the trip to the resort.
If these things don't happen, then I'm not tipping at 20%.