I use a simple seat post mounted cargo rack. Similar to this with just a simple trunk bag strapped to it. I am able to leave shoes and some clothes at work, so I don't have to carry much back and forth on a regular basis. I use at Target bag to put my stuff in and then put it in the trunk bag. That makes it easier to grab and go when I get to work or back home.

Prior to having this I used a backpack. It was ok in the winter and cooler parts of fall and spring, but like others mentioned, it caused too much sweat in the summer.