My very first production sound gig was a 48hr film project a few years ago. And I learned quite a bit making an embarrassing mistake.
First thing I did was talk to a director and get a rough idea of what he wanted to do. What story elements or props, etc. For example I knew a gun might be in it. So in my down time I recorded gun handling sounds, before I even had a script. My goal was to constantly be using my time in a productive manner. A lot of what I recorded I didn't use, but there was a few things I was able too.
During production, the very first location I actually made two mistakes. We started doing sync sound and I tried to record into my computer while at the at the same time sending a signal to the camera. I thought I should at least record to the camera as a backup. So the first mistake I realized was that with all the camera moves it would take to much time to move my comp around too. So the next location I dropped recording to the computer, and we decided to record strait to the camera. Our mobility was quicker and easier that way.
The second, and biggest mistake at that location I found in post. First the editor started cutting things together before he got the dialogue from me. So the scene was cut with the audio recorded to the camera, which thanks to all my signal routing managed to pick up a radio station that was broadcasting a few blocks away. Whoops. So instead of the editor re-cutting and syncing my audio I had to spend a few hours in the middle of the night cutting and syncing my clean dialogue with how the editor cut the scene, all by ear.
After that the rest of the shoot was great, thanks to location and weather I had no problems recording clean dialogue. Also I made a point to record during every scene, dialogue or no. With no time for foley in post I wanted to make sure I had as much sounds and I could.
During editing I sat with the editor and helped him place sound effects where they needed to be and when we needed a sound I would quickly recorded it and bring it back to him to put in.
Most important thing I learned was to be flexible and creative, you may need to adapt in the middle of the shoot. Be observant and take advantage of any little opportunity to grab a sound. Also, remain positive and focused, not easy in the 30th hour, but you don't want to be the crew member that stresses out an already tired and stretched crew.
Good luck. http://vimeo.com/m/#/11916256