I rode my road bike with clipless pedals (the Look variety) and tennis shoes for a month before I tried clipping in. I wanted to get used to the bike handling and speed before I did anything else. Fortuneately when I did eventually clip in, I did pretty well the first time. I practiced clipping and unclipping first on one side then the other as I was riding down a level street with no traffic. After that, I fell at a couple of stops- either didn't get unclipped fast enough or was going so slowly up hill that when I stopped I couldn't get going again by clipping in. I have since learned that on a hill if I have to stop, better to go down a bit and then clip in- little trick of the trade!

So moral of the story- it takes a lot of practice, but once you've mastered it, you'll love the outcome!