Brown your meat in a heavy skillet first. This will caramelize the meat's natural sugars for rich color and flavor. Once the ingredients are golden, add broth or wine to dissolve the flavorful brown bits left in the bottom of the pan. Add this to the slow cooker as the foundation for your sauce.
Give your vegetables a fighting chance. Onions, carrots and potatoes can take the heat and duration of slow-cooking, but their tender green cousins are a more sensitive bunch. Keep the life and color in your peas, fresh beans and leafy greens by adding them during the last half hour of cooking.
Make a good gravy. Slow-cooker recipes require little or no liquid. (the condensations from the machine and the juices from the meat form a braising liquid.) Once the cooking's done, improve this natural gravy by discarding the surface fat and transferring the liquid to a saucepan to simmer and reduce.