Greens tend to fall into two categories: the tender and mild flavored (spinach, chard, beet greens) and the tougher (kale, mustard, turnip and collards). Boiling or steaming the tender varieties washes out the flavors and adds unnecessary moisture to greens with high moisture content. Better to sauté damp greens in olive oil, aromatics and spices. They wilt in a few minutes then remove the lid and allow the remaining liquid to evaporate. For the tougher varieties it is best to stem the leaves which benefit from actually cooking them in water. Helps to remove some of the bitter flavor and produces a better texture. Sautéeing doesn't work because they have a low moisture content and burn before they wilt. Use about 2 quarts of water for about 2lbs of greens (enough for 4) and cook for a full 7 minutes. This will reduce the bitterness without entirely eliminating it as well as produce good texture and color.

Thanks to Cook's Illustrated Best American Side dishes cookbook for this tip.