Traditional Chinese Medicine looks to the energetics not only of food, but also of food preparation. Adherents believe that how the food is prepared is as important as the food itself for how it will be received by the body when consumed. The life force energy we impart to food while preparing it or qi can add or subtract from the inherent qi of the food. Ideally your food is fresh and ripe, pleasing to the eye, smells wonderful, and is delicious before ever doing a thing to it. Examples to me would be ripe peaches, berries, tomatoes, or cucumbers from the garden or local farmer's market. It's beauty and essence feed you before you ever take a bite.
Seasonal fruits, raw foods, and sprouts are most cooling, but should be washed well to avoid parasitic infection. If possible soak all foods to be eaten raw in a mild solution of apple cider vinegar for 15 minutes - one tablespoon of vinegar/gallon of water or 1/4 cup to a sink full of water. Hydrogen peroxide also works - use 1 tablespoon of 3% HP/gallon of soak water and allow to soak 20 minutes.
All cooking imparts some warming quality to foods, but will not change a cooling food to a warming one. Blanching is a little involved, but involves plunging raw vegetables into boiling water for a short time and then plunged into ice water to halt cooking. It basically takes the raw off the vegetable. Steaming adds a moist quality to foods and involves a short cooking time. You don't want to over do it. Vegetables like green beans take about 10 minutes where denser vegetables like beets or sweet potatoes can take up to 30 minutes depending on the size. Cut into smaller pieces for faster cooking. Water saute gives a watery quality and has a quicker cook time than steaming. Cook your vegetables in a small amount of water, instead of oil, and simmer until brightly colored and aproaching tenderness. Boiling with plentiful water and salt for a short amount of time reduces the heating energy of foods. Save your boiling water to cook with other things or make broth to drink. The waterless method allows vegetables to cook in their own juices. Add a small amount of water to the pan to provide steam allowing the vegetables to begin releasing their own juices. Reduce the heat to low and cover for a few minutes, until just tender and bright colored. Salting and pickling in brine adds a cooling effect. Fermented foods like pickles and sauerkraut are cooling foods. Salt has a cool thermal nature.