This is a tough situation for riding in traffic, particularly if there is no turn lane. If there is a turn lane, I would get to the left and into the lane as soon as there is an opening in the traffic. If not, you just have to keep going and wait until there is a good opening. Personally, I don't like riding in the left lane for very long unless the traffic is clear. Drivers just don't expect a bike to be there and get ticked off even if they see you.

I have a situation like that on my way home, and what I have done is change my route. The left turn in question has no turn lane and it is hard finding an opening to merge left. So instead I continue straight on that road another 1/4-mile where there is a turn lane, and make my turn there. I end up taking some different streets that end up in the same place in about the same distance, but I avoid that nasty left turn.

Also, do you have a very bright rear tail light, like a Planet Bike Superflash? That can help a lot. My Superflash is so bright that it is very visible even in broad daylight. If you don't have something like that, you need it.