One of my (many, many) takeaways from this year's Aim Fast, Hit Fast was an explanation for why I never seem to bungle the slidelock manipulation on an emergency reload. (The whole getting-the-mag-in-the-gun bit is another can of worms entirely.)
It turns out that proper procedure (assuming the gun is configured in a manner that allows it) is to ride the thumb on the slidelock lever during the reload. When you slam a new magazine home, the gun will bump upwards into your thumb, depressing the lever at exactly the right time, every time. Satisfaction!
Actually, it hung me up worse at first to find out what was going on, as I accidentally manipulated the lever several times trying to consciously reproduce the effect. But damn if it doesn't work extremely well. Unfortunately the 1911 jockeys are SOL (unless you've desecrated your gun with an extended slide release, in which case I can't help your immortal soul).