In reply to Tyler:
Just go with something that fits comfortably in the shop, and after walking around the house on them for a couple of hours. Most good shops will have a fairly sympathetic returns policy as long as shoes have not been worn outside.
If you're doubting between two sizes that seem equally comfortable, go for the bigger one.
I tend to wear the same size in sneakers/trainers as I wear in mountain boots, for what it's worth (depends a bit on brand though).