One of the drills I teach my students is the smooth and safe transition from shotgun to pistol. Too many people forget some of the basics when it comes to this. Just like in most tactical shooting, slow is smooth, and smooth is fast. Lefties usually make better transitions than righties. Remember to exagerate your movement when swinging your shotgun to the back to give it lots of room to clear gear and it also positions your hand to properly and firmly grip your pistol and bring to the ready position.
OSS, I try slinging the shotty on both sides. I like to try different slings and love to watch other people and their styles.
The last time I went I had a few guys laughing, I have a damaged index finger and have gotten into the habit of keeping it along the slide on the pistol and the reciever on the shotgun and engaging the trigger with my middle finger. Funny thing is, I shoot better like this.