This is the way i do it:
I typically carry a G29 w/15rnd mag, A&G grip extender, and a modified Grams +4 extender on the end of all that. This gives me a lot to grab on to in a hurry, 20 rnds with one up the pipe, and it also works as a counter balance which pulls the gun all the way down into my hand. When i carry my G20, i also use the mag extenders, either a Grams, or a +2, and works basically the same. Everything is wrapped in AGrip (of course) which eliminates any sweaty hand problems, and the mag bases are also wrapped not just for grip control, but so they don't get torn off the bottom of the mags.
Ok, with all that out of the way, the advantage of the extended mag base wrapped with AGrip allows me the reliable ability to first grab the mag base and twist and pull it so that when i twist it the guns mag release button is pressed into my thumb, rather than my thumb pressing in the mag release. When the mag is pulled free i automatically (from practice) adjust the slight change in my grip back to where it's supposed to be, slap in the fresh mag (that's waiting between my fingers), "hit" the base of the mag into the gun with the heel of my hand and give it a little tug to make sure it's in good, (rack the slide if needed) then the gun goes back into action and i put the pulled mag into a pocket.
This is all quite fast as it basically boils down to "grab, slap, hit, tug, (rack slide if needed), boom boom boom..." The mag basically goes into a pocket "first chance i get". (No, i don't spit my mags out on the ground.) If my survival requires that i drop the mag... then i drop the mag... otherwise i hang on to it and it goes into a pocket first chance i get. Usually, that is right after i tug on the fresh mag in the gun and as the gun goes back into action. I use an extended Glock slide catch/release lever and if the slide is already back and i hit it, i hit it, if i don't, i grab the rear of the slide palm down, pull and release it and move the gun into action. I can rack the slide with the pulled mag in my hand. Weak hand (left hand) i grab the mag and twist so that the mag release presses into the side of my left trigger finger or middle finger. The rest is pretty much the same depending on "what i have to work with".