Weirdly enough, I never had a problem with SPD-SL pedals, but when I put SPD/platform (one side SPD, one side platform) on my beater, it seemed a lot harder to clip in for me. But after a number of rides it eventually became second nature too.

In addition to the tips above, remember that you can always pedal with one leg if you don't manage to engage your second foot right off. Also if you relax and avoid being in a hurry, it makes you less likely to mess up the clip-in.