If I cook roast potatoes for Sunday Lunch, I first of all parboil the potatoes for five minutes, then drain and shake them for a moment or two. I grease a baking tray (as though I were going to make a cake on it - I use a little oil and a piece of kitchen paper, but I expect you could use cooking spray), and put the drained potatoes on the tray. Some people (even low-fat types) want to add a LITTLE more oil to the potatoes at this stage, but I don't bother - now I've got some cooking spray, I might give them a sloosh of it, though.
I then cook these for about 45 minutes in a moderate oven. You can also make chips (fries) like this, although I usually use bought oven chips.