I'm going to make some sweet potato fries here shortly and I'm using a technique that I saw on television. I cut up my sweet potatoes into fries and I have been letting them soak in water. My intent is that some of the starch will dissolve into the water and help the fries crisp up better. Has anyone else tried this?
This is a common technique and should work well. It is important that you dry the fries thoroughly before adding them to the oil, otherwise it will boil out of the pan.
If you have the time, you can lay the fries on a rack and place in the fridge for a couple of hours, which dehydrates the surface nicely. If not, pat them dry with lots of kitchen paper.