If you have a trainer, hook the bike up to the trainer to practice clipping and unclipping. No need to hold on to a lamp, the wall, or whatever you were planning on holding onto when practicing.

Another point, just as with regular pedals, the bike is easier to balance when in forward motion, so trying to clip in and out while in place can be awkward, where as doing it when actually riding will give you the proper feel for balancing when unclipping.

Don't worry... 98% of us have done the slow-motion fall when forgetting to unclip. Hurts the ego more than anything else. Just get back up and start riding.