Wear eyeglasses. The VF in my Nikon DSLR is absolutely horrendous. Can't see the frame corners! Have to move from side to side just to see them.
I chalk that up to incompetent engineers (Nikon has a monopoly on those).
It's the same with my Canon 30D. It's just a limitation created by the gap created between the eyeglasses and the viewfinder. I remember it wasn't as bad back in the day when my glasses were rounded and I was able to stick my face a little closer. Moving on to something with a larger viewfinder helps thou, but that's usually tied to sensor size. FF will usually have a larger viewfinder than aps-c dslr, and medium format will have ridiculous large viewfinders compared to aps-c and ff, hassleblad 500c has a pretty neat viewfinder.