I'm sure many of you have already said this, but really the only way to practice initially is to do it in your driveway several times standing over the bike so it becomes muscle memory on each foot. Then try getting in and out randomly when you are actually riding when there are no stops to get the feel of what it is like to clip in when moving. I try to anticipate stops as best I can, but sometimes it's unavoidable and you have to trust your instincts to clip out. My dad rode with me one time from my apartment and we decided to try a different route that involved us having to ride through many stop lights and intersections. He told me after the ride since he was very new to clipless pedals after I convinced him about their advandages that he had more practice that ride than where he normally goes. Consequently he said he felt much more comfortable getting in and out of his pedals with the real-time scenarios that forced him to clip in and out frequently. I like to push off with my non-dominant foot so I can get into my pedals better as my right leg is my dominant one.