The best way is to look for the pink shirt guys...they belong to a taxi co. that caters to tourist...
They are very knowledgable about the area, very courteus, and they should only charge you $30 USD one way to the Bahia Principe from the airport in Punta Cana...
I know...I've done it before...
Have fun...the place is beautiful (the hotel) good food, nice, new clean rooms...we had an awesome time...and plan on going back REALLY soon!!!!