Jorde makes some excellent points.
Steady lights make it easier to identify some problems before shooting.

Also, give plenty of attention to balancing your illumination. Meter the center, corners, top, bottom, left, right and adjust the lights to get the readings as close as possible. I like to use a flat disk on an incident meter but a reflected meter and a grey card works just as well. Uneveness that is almost imperceptable by eye can be very noticable in the slide.