When you're practicing drawing your sidearm, there's a natural inclination, when you bobble the grip or pull or whatever, to stop, holster, and try it again.
Finish the draw. No matter how badly you mess it up, finish the draw. If things go to hell and you grab, you get no chance to do it over, so get used to correcting as you go. If your grip isn't perfect, if your draw isn't quite right, if your shirt or jacket gets in the way, whether you're using a pocket carry or belt or in-the-waistband holster or shoulder rig or whatever, finish the draw. Fix it as you go, every time.
If you ever need it for real, pretty doesn't matter, perfect doesn't matter: getting the sidearm out, aimed and speaking accurately does. So get used to things going wrong. Finish the draw, every time.
In the comments Panday said something very, very true: As the old saying goes, "Slow is smooth, and smooth is fast".
Oh, yeah. I should have specified in the original post, "You start learning this slowly. Reach, get a correct grip, draw, etc.... Once you have the motions down, once you understand what you're doing and how and you start working on speed..." Beginning any new physical activity you start slow.
The most amazing pistol man I've ever personally seen was Capt. Dan Combs of the OHP, who I've mentioned before. Spooky-fast and accurate(with damn near anything). He said that when starting this he began by moving slowly, making sure he had each motion right, and once he had that locked in he began speeding up. The time I saw him he would hold a foam coffee cup on the front lip of his holster, flip it down to the ground and draw & blow it up before it hit. Then, commenting that "Trooper ( ) says that's just luck, so let's try it again," would do it again. Only this time he'd deliberately miss the first shot and blow the cup with the second, still before it hit the ground. And he did this with a S&W Model 19(polished action, no other mods) and a standard-issue Sam Brown belt & holster.
Start slow and get it right. Then, when working on speed and positions, finish the draw.