Peel the sweet potatoes (not strictly necessary, but I prefer it. If you are going to leave the skins on to preserve all the nutrients in the skin, use very fresh sweet potatoes, trim, scrub, and dry before cutting).
Cut the sweet potatoes in half with a sharp chef's knife. If they are large, cut each half lengthwise into slabs thin enough for fries. Lay halves or slabs flat and cross-cut into strips ¼" or less thick. Cut any extremely thin ends off the fries to prevent the ends from burning (alternatively, cut the pointy ends off the sweet potatoes before cutting them in half).
Put the sweet potato fries in a large bowl.
Add oil and/or egg whites, salt, and pepper to the fries and toss.
Cover two baking sheets with parchment paper.
Spray the parchment paper with olive oil from a cooking spray can.
Arrange the fries in a single layer on the pans. Don't crowd too many on one pan, it will make it harder for them to become crisp.
Heat oven to 425 on convection setting, if your oven has one. If you have a conventional oven, you may want to bake at 450, but if you do, watch the fries very carefully.
Don't bake anything else in the oven at the same time, that will slow the fries down and make them less crisp.
Bake, checking and stirring every 10 minutes for 20 minutes, then every five minutes.
They should be done in 20 to 25 minutes.
Recipe by Fit and Fed at http://fitandfed.net/2012/02/17/baked-sweet-potato-fries/