I have to confess, great sweet potato fries are something that have always evaded me. The great ones we have in diners and bars are particle composites, or sweet potato plywood, not slivers of the veggie itself.
A few things have gotten me close though, albeit never in the oven, but in a vat of oil.
The thing about it is this: what we know as crispy is a result of the exterior of an ingredient quickly dehydrating from an extreme heat. Sweet potatoes contain too much water for this to happen very easily at a low temp(it never crisps), and too much sugar for it to happen at a high temp. (it burns).
If you're dead set on using the oven, I'd cut them small (shoestring size) and use a decent amount of oil, essentially frying them in the oven.
In the frier, I par-fry them at 325 degrees until soft. Then I toss them in a flour and cornstarch mixture before refrying at 375-400 degrees. It is this coating that gives them crispness, not the sweet potato itself.
