The ultimate tip might be to avoid turning on your oven. Ours takes a long time to cool down and is the number one culprit in heating up our little kitchen. If we're baking a cake or cookies, we try to do it early in the day, so the kitchen has time to cool off before we have to be in there for an extended period of time, making dinner.
We also find that opening our kitchen window isn't enough; we need the airflow from one room to the next, so we have to open a window in the living room to get things circulating.
But we're eyeing a pretty, old-fashioned fan for the countertop. Something like the one in this charming photo of Eudora Welty's kitchen in Mississippi:
So what do you do? Do you have any tips for keeping the kitchen cool and comfortable?