Are you using a yeast starter? Aeration and yeast activity go hand in hand in the early stages.
If you use a yeast starter, a little aeration goes a very long way. The yeast cells have already reproduced many times in the starter, so you have a huge army of yeast cells that can rip into the sugars almost immediately when they hit the wort. With a decent size starter, you probably wouldn't need any additional aeration at all.
If you don't use a starter, you should try to aerate frequently during the lag phase when the yeasties are trying to reproduce. They can't begin the rapid fermentation phase until they boost up their population quite a bit, and yeast reproduction is where the oxygen comes into play. Shake your fermenter into a froth before pitching the yeast, and then give it a shake every couple hours (or whenever it's convenient) until you see the krauesen starting to build.
