My commute is 18 miles one-way. First, get gatorskins, you'll be glad you did - trust me.

After the first summer with a back pack, I got a full blown commuting specific bike with racks, panniers, disc brakes and schwalbe marathon plus tires. Since it was getting dark earlier and daylight savings approaching I went with a full battery of lights. Mainly to be seen, not to see.

Also - I had a messenger bag. I couldn't get the load balanced very well so I went back to the back pack.

Trunk bags don't hold enough. I also bought a burley nomad trailer and found it weighed more than the bike and was a general PIA to use for commuting.

I have now come full circle and pack 1 bag for the week and take it in on Monday. (I usually commute no more than twice a week.) I ride in on my road bike with no bags at all. It's much faster and enjoyable than lugging all my clothes.

You can learn as you go as I did and see what works best for you.

Oh yes, and this belongs in the commuting forum. Good luck.