Raw sweet potatoes are mostly starchy fibers, but they also contain an enzyme that will break down the starch into sugar at certain temperatures. According to food scientist Harold McGee, this enzyme happens to work best when the sweet potato is between 135°F and 170°F.
Logically, the longer you can keep the sweet potato within this temperature range, the more starch will get converted to sugar. Steaming and boiling cook the sweet potato too quickly, but baking it in the oven at 350°F or lower is perfect. Sweet potatoes are done when they're soft all the way through.
If you're using the sweet potatoes in something like a casserole or a pie, you can bake the whole potatoes ahead of time and keep them for several days before using them in your recipe.
Give it a try!
(Oh, and by the way, that photo up top is from Pithy & Cleaver, where you can find this recipe: Sweet Potato Purée with Pecan Streusel. Yum!)