Before painting a door
Remove all handles etc before starting and lay newspaper under the door to prevent your brush picking up dust.
Wedge the door in an open position to hold it steady while painting.
Which part of the door frame to paint
Many people are confused as to which part of the frame belongs to which room when painting. The answer is that all frame edges up to and including the ones against which the door closes belong to the room into which it opens.
It is generally better to paint the frame before the door. Painting it afterwards will inevitably lead to the brush marking the part dry paint on the door.
If in doubt, let one dry before painting the other.
Painting door edges
Door edges must always be painted before the face for the same reason.
When painting the face of a door, work slightly outwards at the edges rather than inwards to avoid what the trade call 'fat edges'