Motoconchos in Sosua are inexpensive, and my recommended method of travel within Sosua, so long as you know what the price is and let them know what you are going to pay BEFORE you get on. 5pesos anywhere in town during the day (per person) and 10pesos at night. Taxis are more expensive, but safer for the longer hauls, especially at night. Don't take a concho on the highway (to Cabarete, for example), night or day.
Travelling to Cabarete/Puerto Plata is best done on a guagua or publico during the day. Guagua is 5pesos to Cabarete, 10pesos to PP. Publico is 10 and 15 pesos during the day, 15 and 20 at night. Trouble with public transit is that you have to wait on the side of the road...which is ok in the day but can be scary at night.
"Real" taxis in Sosua are parked at taxi stands, not on the side of the highway near the Texaco. The real ones are beige, and in good condition...fairly easy to spot. Like AmyG said, get a business card if you like, and you can use the same one for most of your trip if you want.
Have a great trip!
marc