Selecting: Choose evenly-shaped sweet potatoes with firm, smooth skin. Avoid sweet potatoes that are bruised or blemished or that have white areas, as they will spoil faster. Small or medium sweet potatoes are creamier than large ones (which are starchier).
 
Preparing: There are many ways to prepare sweet potatoes, but baking whole is highly recommended. Pierce whole sweet potatoes with a fork or paring knife. Bake at 400° F until tender, 45 to 50 minutes. You can split open a baked sweet potato, top with chili or some other stew, and make it a meal.
 
Steaming is preferable to boiling to preserve maximum nutritional benefits. Sliced into ½-inch slices, sweet potatoes can be steamed in just 7 minutes. Once cooked, you can drizzle with olive oil and other seasonings and serve. Mash or puree the cooked slices for a different texture.
 
Storing: Sweet potatoes are typically cured before you buy them, which allows cuts in the roots to heal, protecting them for longer storage. Cured sweet potatoes can be stored for several months. Individually wrap each sweet potato in clean newsprint or brown paper bags. These materials allow the potato to continue to breathe. Place the wrapped potatoes in an open box or basket to be kept in a dark cool location (between 55 and 60F). A root cellar is perfect for this. Under ideal conditions, sweet potatoes can be stored for up to 6 months. Raw sweet potatoes should not be refrigerated because it will accelerate deterioration.
 
Freezing: Cooked sweet potatoes can be frozen. Wash and peel the sweet potatoes and boil them whole until tender, about 15-20 minutes. You can slice or mash the cooked sweet potatoes. (Freezing them whole is not recommended because the texture will be altered after thawing.) Sprinkle the prepared sweet potatoes with 1 teaspoon lemon per potato. The lemon juice prevents discoloration. Let them cool. Freeze in airtight plastic bags or containers. Sweet potatoes can be frozen for 10-12 months.
 
Sources:
WikiHow
The World's Healthiest Foods