Hi I saw this post and thought I would post a reply that I had put in another thread for newbie riders since I didn't see this suggestion in this thread. I got this tip I got from a sales associate at REI.

Bring your bike inside, and place it in a door frame (yes, the door should be open). Mount your bicycle and maintain your balance using your hands against the door jamb. Practice clipping out of your pedals while stationary in the door jamb. You'll be able to get about 40-50 practice clip outs in about 15 - 20 minutes.

The best part is you will be able to decide quickly whether you want to loosen the clips tension or not and you will get a lot of experience quickly. Your confidence will rapidly increase and you will go faster longer.

I changed pedals, when I bought a new bike, I went through the same routine again and it was well worth the time. At the risk of jinxing myself, by doing this exercise, using both pedals, I have never fallen over.

I hope that helps.