This is gana sound bad coming from me (the resident panel beater) but I still find them hard some times too. Some times what i do to make it easy is I will use packers in the gaps so that its the same gap top to bottom or side to side.
Some times you need to average it out as well (basically hang it so that if there are inconsistancies, they sort of blend out or are less easy to notice).
The other thing that will help you, get a trolley jack with a few folded rags on it to support the door while its open (just while the nuts are loose). You can buy special door stands but a jack is just as effective.
One other thing, so long as the gaps on each edge are the same the whole way alone their edge its ok. Even if one edge is 5mm and another is 8mm or what ever. What the eye is more likely to notice is if the gap closes up at one end.
Anyway, I hope that helps a little... if your still stuck let everyone know
