I never had any issues with the impressions, have gotten them done several times for braces, retainers, night guards...last year, I was getting them for a night guard, and the stupid assistant kept messing up until the dentist came in, and they had to do it like FOUR TIMES on the upper part! gahhh... -_-

Basically, you just need to lift your soft palate and open wide, kind of like when you are singing.