A light is probably the most important accessory. The Surefire foreend is expensive, but very nice.
Even if you aren't in an "extended gun battle" with a shotgun, its nice to have the side-saddle, because it give you the option of switching to slugs if you need to. If, God forbid, one of your loved ones are in the hands of a bad guy, how willing are you to trust your ability to offset your buckshot pattern enough to miss your loved one, and hit the bad guy? With slugs as an option, this isn't an issue. Plus, they sure do a lot of damage.
Stay away from butt-stock sleeve ammo holders. If you have to shoot left-handed (and the world has just as many left-handed corners as right-handed corners), these make it much more difficult.
Stick with a very simple sling, with a large loop hanging down. The only thing you need the sling for is to retain your gun when you need to use both hands. Single-point and other "tactical slings" seem like a swell idea, until you have to actually perform a physical task (like carrying a wounded person) while using one of these slings. With a standard sling, or a three-point sling set-up with just a large loop, its a simple task to sling your gun across your back, muzzle up or down. Your hands are now completely free to perform necessary tasks, and your gun isn't swinging around, bouncing off of things (like the body of the person you're dragging).
The wound ballistics experts recommend standard loads of #1 buckshot. Whatever ammo you use, make sure it works flawlessly in your gun. Some european ammo doesn't work so well in Mossbergs (S&B Buckshot, Fiocci Buckshot).
If this is going to be your primary home defense gun, keep it loaded with the safety on. Although some people think it would be cool to scare a bad guy with the sound of a round being jacked into a chamber, I'm not interesting in scaring a bad guy, I'm interested in eliminating him as a threat. If you have to chamber a round, the bad guy will hear it and know that there is an armed defender in the house. This may make him much more agressive / dangerous. If I were ever in a home defense situation, the first indicator that there is an armed defender in the home that I want to give a bad guy, is the high intensity light shining in his face. The second indicator will be the "bang", after he's been positively identified as a threat that needs to be neutralized.
I have two 500's and a 590. They are excellent guns.
Now get out there and shoot. Better yet, take a Tactical Shotgun class. It will be an eye opener.