I'm with FFM. If the water I am fishing allows a 2 fly rig I will be fishing one if I am nymphing. To add to the list of tips that have been given so far I will add this. If your first fly is barbless do not tie your dropper onto that fly at the hook, it will slip off. If you are fishing a barbless hook as your top fly I would suggest tying your dropper to the eye of the top fly, it can be a pain if the eye is small but it will stay on.
The way I look at dropper fishing is that in a lot of water we fish we have to add weight to a one fly rig and that weight (split shot, putty, etc) does not have a hook and does not look like a bug! It kind of drives me crazy to put enough shot on the line to get a smaller fly to the bottom. Having said that though a lot of the time I still have to add weight above the top fly.
Brian