I"ve been doing clipless for the last week and a half and I'll try my best to pass along a bit of advice of what I've learned thus far -

1) Better to unclip earlier than later - esp when starting out
2) I usually prepare to unclip one foot when I begin to coast to a stop (for a stop light or stop sign) - usually when my speed drops to around 10mph or so I'd say
3) I usually bring my clipped foot up to around 2 o'clock for a good kick start from a stop, and begin clipping in the other as the unclipped pedal hits 12 o'clock. I get my foot in position on the pedal and press forward/down with the down stroke to engage the cleat in the pedal.
4) Learn to always unclip one foot or the other - and stick with that foot. I knew from previous rides that I always stepped down with my left foot first and mentally reminded myself of that the first few stops clipped in. After that it was just second nature to do so.
5) Unclipping one foot - usually around 6-ish. I just let my leg fall naturally to the bottom of the stroke and then kick my heal out to disengage.

Good luck - and remember rule number 1.