Here's what I do ...
Slice the sweet potatoes in julianne strips (like french fries). Then I put them in a bowl and toss them with a couple tablespoons of olive oil.
Next, place them on a pan (one even layer) and put them in the freezer for 20 minutes. (I do this in bulk, and store in the freezer - in ziplocks - what I don't need for future recipes.)
Then, heat your oven to 400 Bake for 30 minutes (a hot oven is essential to crispy fries). Put your fries in the oven on a baking dish. Bake for 10 minutes and turn the fries ... then 10 minutes more.
Enjoy!