I have discovered it to be helpful to sit down and make a menu each week for dinners prior to my weekly grocery shopping. Not only does this cut down on extraneous/impulse purchases while at the grocery store, it saves the refrigerator from becoming overly full and unused food from rotting. This way one only buys what one will use for the meals each week. It also keeps our weekly grocery bill extremely consistent (around $80-90 for 2.5 people).
Once you make the menus, then you go through and write down the ingredients that you don't already have, and add a couple of staples for breakfast and lunch.
To help out with this RealSimple has made a cute (and actually helpful) line of organizational home tools.