We usually tip about 20% breaking the tip about in half for the two trips. We've always had two different drivers, so although you have to pay for the entire trip on the front end, I don't recommend doing a complete tip on the front end.
Also, we tend to tip our driver from the airport to the resort a little better than for the return trip. We do that because that driver does quite a bit more for us -- helping with bags from baggage claim, waiting during the grocery stop, and psyching us up for the vacation. The trip to the airport is usually subdued and all the driver needs to do is drop us at the curb.
That's my two cents!