Quite simply: buy eggplants as fresh as possible and use them right away.
The older an eggplant gets, meaning the more time that's passed since harvesting, the more bitter it becomes. This explains why the eggplants we buy at the farmer's market this time of year are always so delicious while the ones we buy at the grocery store end up with problems. Who knows how long ago those grocery store eggplants were picked!
Of course, if grocery store eggplants are your only option (because who doesn't crave a little eggplant parmesan in the middle of winter?) then it's time to try out those other remedies. We've had the most success salting the cut cubes of eggplant and letting them sit for a few minutes while we prep the rest of the recipe. Harold McGee in On Food and Cooking theorizes that salting doesn't actually change the eggplant's bitterness so much as it decreases our perception of it, but we'll take it if it works.
Any other tips for avoiding bitter eggplant?