WATER
Use as little water as possible when cooking vegetables.  Use the leftover water in soups or to make broths.  Try not to wash vegetables until just before using.  Avoid soaking food in water.  Water dissolves the water soluble vitamins and minerals.  Cooking vegetables in the inset to a steamer pan will keep the vegetables out of contact with water, and cooks them in minimum time so vegetables are tender-crisp.
 

AIR
Air is a big culprit in destroying a host of vitamins - A, C, E, K, the B-vitamins (thiamine, pyridoxine, biotin, and folate or folic acid).  To minimize the vegetables to air, here are some tips that you can do:
- cut and cook vegetables in pieces that are as large as possible
- cover vegetables as soon as they are cut and cover while cooking
- cut vegetables as close to serving time as possible
 

LIGHT
Riboflavin is a vitamin that is sensitive to light.  Exposure to light for just a short time allows a good portion of the riboflavin to destroy.  That is why you will often see milk now packaged in yellow containers to shield milk (an excellent source of riboflavin).  Put milk back in the refrigerator immediately after pouring it.

HEATING FOODS
Vitamin C is especially sensitive to heat.  That is why frozen vegetables are generally higher in vitamin C than canned.  The canning process destroys some of the vitamin C.  This is another reason not to overcook vegetables.

PREPARING PASTA, RICE, AND OTHER GRAINS
Rinsing pasta, rice, and grains before cooking washes off the vitamins and minerals that are sprayed on to enrich the rice.   Many recipes call for browning uncooked rice granules, especially arborio or basmati rice in olive oil.  This destroys the thiamin (B1).

ALKALINE PH
You have probably heard that adding baking soda to vegetables such as green beans keeps the green color bright.  The down side is that it makes the water alkaline and destroys thiamin (B1) and vitamin C.