There's really two problems: the first is initial fit. Initial fit can be solved if the company you order them from has a good return policy. Some companies will even let you order more than one pair a half size either way, knowing that you're going to return one of them. Course, you could be returning both of them if both are too tight or too loose someplace.
The second problem is how they fit after you have had them on several hours in working conditions, when your feet begin to swell and get tender. I'll have to admit that I've been stung on boots that I thought fit fine only to find out that they don't after several hours on the trail. The only advice I can give you on this second problem is that if there is any little thing that doesn't seem right when you're trying them on, it's probably not going to get better. It's your job at that point to be super critical, like a woman picking out wallpaper, not like a man deciding what tie to wear with what shirt.