Great oven fries can mimic, if not entirely duplicate, the best qualities of their deep-fried cousins—golden, with a crisp exterior and fluffy middle—yet remain much lower in fat. Bad oven fries, however, can turn out pale and soggy, or dry up and burn, sometimes achieving both states in the same batch.
The solution: It seems counterintuitive, but you need to presoak. Nearly half a potato's weight is accounted for by water. Soaking pulls out starch, which reduces the water content of the potatoes: less water, less steaming in the oven.
Start with baking potatoes (russets): They're drier than waxy varieties. Cut each peeled potato in half lengthwise, halve again, and slice each quarter into ¼-inch-thick strips (a mandoline is nice but not essential). Even thickness and wide surface area prevent burning and give you more crispy real estate. Soak in cold water for 30 minutes, then dry thoroughly with paper towels.
Toss with olive oil, and then spread on a parchment-lined baking sheet. Don't overcrowd the fries, or it will be a steam bath in there. Bake on the bottom rack at 400° for 35 minutes. Flip once halfway through.