where are you staying in Juan dolio? the best way to get from the airport is to have your hotel or someone to arrange an airport pickup for you. Search the threads here..there was one recently about the prices.
The best thing tho is to have this arranged BEFORE you travel!!!!!
THis is a common question for DR1...perhaps the administrators could add a section to the main page to include reputable car/taxi services with rates etc. Just a thought