The US Department of Energy has a whole blog on saving energy, and this post has many suggestions on how to save energy in the kitchen. First, match your cooking vessel to the size of your burner and make sure it touches the burner to get good conduction of heat; heavy pots and pans will conduct heat more efficiently, too. Second, keep a lid on your pots and don't peek (I use glass-lidded cookware for this reason!) In summertime, they suggest using small appliances to cook food - toaster ovens, crock-pots, and microwave ovens. Avoid using the oven in summer, (and when using it in winter, adjust your thermostat to compensate!) use the stove-top instead; a pressure-cooker will save even more energy - and, of course, summertime grilling keeps the heat outdoors where it belongs.
A couple of techniques will help you save energy regardless of the time of year. Maintain your large appliances: vacuum the coils on your refrigerator regularly, and check the seal on the door by trying to slip a dollar bill under it (if it slides out easily, replace the seal.) The seal on your oven needs to be checked as well, and keeping your oven and stove burners clean will also save you energy.