Longer lens defiantly. Also a larger front element is called for as well. (77mm vs. 52mm). But if you don’t have one of those, oh well. If you have a lens hood, use it if you are not already.
Try to keep the camera/focal area as perpendicular as possible to the door itself. If the door is tilted, angle the camera to match that tilt. Also, unless you are doing an emotional image (one of the door at an extreme angle or such) try to center the image at the two thirds point on the door, or at eye level. This gives off a more natural image that people can relate too.
Try to ‘neutralize’ the light reflecting off of the door itself, (a soft box comes in handy here). Also use multiple lighting sources. If you have not that, then kick in the old tungsten lighting and use a filter or adjust the white balance.
When using a longer lens, you have a tendency to loose a bit of depth of field.
Try to angle a light to have a slight bright spot in the middle or at the top BEHIND the door. This will have the effect of giving you more depth with the longer lens. Set the door up (if you can) with more distance between it and the background. You want to try and wash out the back of the door a bit to visually distract the viewer’s eye from the straight line the door makes. The partial line at top and bottom will be compensated by their brains and this will cause them to think the edge is straight. You’ll have to experiment with this, but the results can look a great deal more professional.