The main rule of thumb for healthy cooking is to avoid methods that require excessive fat. I would urge you not to fry food and especially to avoid deep frying, which not only adds lots of calories but also exposes you to the health risks of oxidized fats. However, stir frying is a different story - it allows you to cook foods quickly, combining vegetables and protein. Just use small amounts of good oils (extra-virgin olive or organic expeller-pressed canola) and keep them below the smoking point. (If you want to pan-sear anything, use healthier oils that can tolerate higher heat like grape seed oil.) One technique I like is "steam frying" - that is, sautéing food briefly in a little oil, then adding water, stock or wine and covering the pan. Allow the food to cook until it's almost done, then uncover and boil off any excess liquid.
Broiling, baking and roasting can also be healthy methods of preparation, provided you don't add unnecessary fat.
I often steam vegetables and fish - this is a particularly healthy cooking technique because it does the least damage to nutrients and, as a bonus, lends itself to a quick clean-up. I tend to steam tender vegetables and boil less delicate ones (potatoes, beets, corn on the cob). A rule of thumb is when you smell it, it's done. I like my vegetables when they have a deep color to them and are a bit crunchy.
I enjoy grilling outdoors, but high temperature grilling (and broiling) of foods that contain fat and protein (meat and poultry, especially) produces carcinogenic compounds called heterocyclic amines (HAs) that can raise the risk of colorectal cancer in those with a genetic predisposition to the disease and may increase the risk of other cancers. To protect yourself from exposure to HAs, grill more vegetables and fish and less meat and try not to cook animal foods (including fish) to the point of charring. (Blackened fish is popular but unhealthy - try to avoid the blackened parts if you're served food this way.) If you do grill meats, use leaner cuts and marinate meat, poultry and fish before cooking; the marinade may help reduce HA formation, especially if it's made with spices such as ginger, rosemary and turmeric. And, finally, avoid charcoal lighter fluid or self-starting packages of briquettes in a charcoal grill - they will leave residues of toxic chemicals in your food. A healthy alternative is an inexpensive chimney lighter that uses a small amount of newspaper to ignite a mass of charcoal in a large metal cylinder. Gas grills are good alternatives to those that use charcoal.
Andrew Weil, M.D.