Lots of good advice above, to which I would I add the following points:
1. Forget what you've heard about the Dominican education system; they all know their physics. The larger vehicle always, and i mean ALWAYS, has the right-of-way.
2. Driving there requires a heightened state of awareness. Use the mirrors! You need to know not only what is in front of you, but also what is behind. When passing, check the side mirrors, the car behind you may still be in your rearview, but the 23rd car behind him may be trying to pass all of you. Know whether or not the oncoming lane has a median, oncoming traffic will (usually) slide over to allow you to pass, but only if they have room to do so. Know your own vehicle's acceleration ability. Judge the degree of sobriety of all other drivers. Regard them all as opponents, if not enemies.
3. Control your space. In the case of getting passed while making a left turn, this can easily be avoided by guarding your flank. If you know you need to turn, and also that there is madman flying up your tail end, casually move towards the center of the road, even if it means forcing oncoming traffic to swerve towards the shoulder. This will cause said madman to switch tactics and pass you on the right, thus avoiding an accident. If it suits you, you could even use your turn signal.
4. Use your horn. Whereas in the states the use of the horn could provoke an unpleasant incident, in the DR the failure to use it is equally offensive. In Moca I once had a motoconchista pound on my window (while driving at highway speed) because when the light turned green I waited until traffic stopped and made a left turn, not realizing that the moto was not required to stop at red lights and that I had failed to announce my presence with authority. I could tell by the gestures he was making that I was most certainly in the wrong. Here's how to use the horn:
- One honk, not too short, not too long = "Coming through!"
- 2 honks = "Coming through and I'm doing something a little stupid..."
- 3 or more honks = "Presently I am writing a check for your funeral expenses."
5. Keep some cash handy... you know, just in case.
Hope this helps.
Chris