certain routes ie. transcons, & Iah to EWR are often hard to upgrade and I always burn miles at the time of my res .I also stay away from 757-300 planes as the FC seats have been reduced.I also look at the cost difference between W or X fare ,some of the lowest and a U or higher .If it is minor I buy the more expensive ticket as the raffle between who gets the seat amongst Plats is going to be by the ticket class and price once all else is equal.