Given their comparatively limited capacity, reloading is an important skill for combat shotgunning. When you've only got a few rounds to work with, you need to be fast at topping off in an emergency.
Three gun and cowboy action shooting, both prominently featuring scatter guns, have helped push the envelope in terms of shell carrying gear and reloading techniques.
Here are a bunch of YouTube videos to showcase some of the techniques. All of these will require special shotshell carriers and lots of practice to get any good at--but that's part of the fun, right?
Some of these are cool but less useful outside of competition - keeping eyes up on target and maintaining mobility are important. And, most competition gear is not designed to stand up to the rigors of combat - roll down a hill with some of these and you'll have shotgun shells all over the place.
Pull 1/Load 1
About 48 seconds in. One round at a time, via the port. You can do this from belt pouches, your stock, or a side saddle.
Side Saddle Combat Load
About 3:40 in. Same basic technique as above, but from a side saddle.
Yaeger vid, so there's probably some colorful language at some point :)
Pull 2 - Tube Load
Pull a Whole Bunch
Pump stuff. About 1:55 in. Double barrels at 5:50, too.
Caddy/Weak Hand Reload vs. Load 2/Twins
Reloading techniques compared.
Caddy/Weak Hand Reload - More Detail
Load 2/Twins - More Detail
See! This stuff takes practice.
Speed Loader Stick
Requires a mod to the shotgun, too.
How do you load your shotgun?