Without knowing what equipment you are using thats a difficult question to answer.
Personally, I shoot in AV mode (Canon 40D and Canon 7D).
In the mornings, I shoot F4 or lower,ISO 500-1000 and the camera sets the shutter speed. As the day brightens up, I dial the ISO down (320-500) and close the aperature down a bit (f4-f16 depending on lighting conditions).
In AV mode (Aperature Value or Aperature Priority) mode,the camera picks the Shutter Speed according to the other settings I mentioned.
In TV mode (Time Value),you set the shutter speed and the camera sets the Aperature in accordance with the ISO.
These are the two modes I use most,and, I use AV most of the time
BUCK OFAMA IN 2012