"Go for the largest size that doesn't give more than a minimal and completely benign amount of heel slip. Your feet get bigger on a high-mileage day and more so on long trip. That's almost certainly a 1/2 size larger than you usually buy, often a full size larger."
David, I've heard that mentioned a few times but there is also the view that its the upper aspect of your feet which become larger with use not the lower part which is the heel fit, so I've heard simple loosen the laces after walking for a while will do better? That trick has helped me out I just redo the laces after a few hours and it makes feet comfier at the end of the day, and costs $ nothing.
Anyhow, I'm trying for the first time some Roclite 315. Nervous about the change but so many +ve comments I'm giving a shot.
The advice of once you find something which works, buy multiple pairs, is very good advice, your feet don't change much over years but manufacturers "improve" their designs almost every year. Also each shoe will wear in some spot first, so if that is somewhere on the sole then the next pair, proactive apply some Shoe Goo to that spot and it will even out the wear and give a bit more life. In my case its often the heel outside edge wears first and that accelerates the shoe's failure so I now apply Shoe Goo to new shoes on that spot and replenish, it almost doubles the shoe's life in my case, til at least some other part then wears.