I second Paperwing's endorsement of panniers. Also, put a chain tool in your toolset in case your chain breaks...you can fix it and keep riding (learn how to do it if you don't know).

I commute 14 miles from Maryland through DC to my office in Virginia. The Virginia part is on a multi-use trail, but the other part is all on streets with traffic. However, I do not go the same way I would go in a car. My rather roundabout bike route is much nicer, quieter, and only 1 mile longer than the car-route. It's also a whole lot safer. So get creative with your route.

Left turns: usually I am on smaller streets, so left turns are not a problem, although I'm careful and signal and take the lane. In one spot, though, I do hop on the sidewalk. It's bridge over RR tracks, so there's no danger of cars pulling out of driveways (a serious hazard of sidewalk riding). The road over the RR tracks is crazy fast and there's not much vertical sight distance.

Clothes: I wear obnoxiously bright clothing. See www.alertshirts.com for some good shirts (see my post under "Sewing Shirts" to see how I'm adapting mine to cycling). I also have good lights, front and back, and reflector things on my ankles. I figure you can't be too bright.

Have fun. It's a great way to improve fitness while not adding any more time.