technique - How does one get manual focus right with a fast-aperture lens?

I have got an old lens (f1.4) that I have to use only with manual focus on a DSLR. Well, it is difficult for me to get the subject on focus. Although on focus according to the viewfinder, the actual picture is blurred or the focus is slightly elsewhere. I usually reduce the aperture to avoid this. Do you have any suggestions?

Added: I'm using a Pentax K10D.