Or you can go through El Malecon - this is the 'old' road. Much quicker way but it is bit more dangerous and populated because this is how the local guaguas go to Haina and San Cristobal.
We usually take the malecon west into San Cristobal. Or, as Talldrink said, take 27 de Febrero west out of Santo Domingo. Signs are pretty clear. Once I get into San Cristobal, that's when I usually get lost :cheeky: