This is going to sound ridiculous, but it worked well for me.

Bear with me (or is it 'bare'? I never know!) it's gonna sound funny.

For a month before I tried clipless, I used imaginary clipless pedals. Yup. Imaginary ones. Every stop and start, I pretended I was clipped in and made myself twist my ankle out, and make sure I got a good solid imaginary clip in from a stop. At first, while I was still getting comfortable on the bike, I would forget. No new pedals for me that week. Until I did my entire ride without a mishap, I didn't think I was ready. When that little 'flick' of my ankle became second nature, that's when I knew it was time.