I was surprised when reading through this thread that nobody suggested the most obvious remedy. That is do a bunch of lead climbs in the gym. New leaders can practice clipping a rope while they are being belayed on a second rope rigged for top rope. Not so new leaders can do the whole deal. You can get a lot more pitches in (and a lot more clipping practice) climbing indoors than you can climbing outdoors. And it's arguably a lot safer and forgiving of an environment than most outdoor crags. In addition to becoming proficient in making the clip, you also want to practice other good safety measures like avoiding back clipping, avoiding z-clipping, and being aware of how your leg is positioned with respect to the rope. Of course these are all things you need to pay attention to while trad climbing too. Most gyms have staff who can help you develop good clipping techniques. Have fun out there!