First off, it's probably best not turn left across 2 lanes of oncoming traffic if you don't have a stop sign or traffic light and there is a lot of traffic. I'm assuming you are in two lane traffic with a stop sign or traffic light.

Make your way left into the middle of the rightmost lane, taking up all of the space you can in that lane so no car can pass you. Look back to find a good gap to your left, and when you get a good gap, get into it and take up the whole lane again. Cars behind you can always pass you on the right at this point even if you have made your way left a little too early. Better safe than sorry. Once you arrive at the left turn, you should be fine to go when the traffic light changes or when people at the stop sign recognize there is a cyclist.

If you know can learn to track stand, I find it helps sometimes. I find that if I do a track stand, cars pay more attention to me and it also lets traffic know that while I don't intend to cut them off, I'm ready to take the right of way as soon as it is my turn. Then if the traffic really should have the right of way, I put a foot down telling them that it is really their turn to go. This clears up any ambiguity which tends to arise with drivers who are too polite.