If marinated, meat should be drained well prior to cooking. Since stir-frying is usually done over relatively high heat, be sure your wok or skillet is large enough to hold food without overcrowding. Otherwise, food will steam. Most ingredients need to be cooked in batches, e.g. 1 pound of beef should always be stir-fried in two batches. Meat and vegetables should be cooked separately, then combined after cooking.
Vegetables can be stir-fried in water instead of oil to keep colors bright and flavors fresh.
Vegetables should be cooked until tender-crisp and still brightly colored. Group vegetables according to time it takes to cook. Begin with those requiring longest time, then add others accordingly.