Shield accessories
I just got a Shield and looking. I have a few Crossbreed IWB and guess that's the way I'll go. They're not very attractive to look at, but VERY comfortable and secure. The only downside is the 4-6 week delivery, but I'll proably have too. Crossbreedholsters.com will get you there and many YouTube videos out there to see it. Google M&P Shield holsters and you'll get a bunch of other ones.
One major consideration is whether you get a laser, the holster may be different depending on the way the laser mounts. The ones that mount in the front require a different holster mold.
Sights, Trijicon is the obvious ones, I've always had, 20+ years. I've been considering the XS Sights, xssights.com, and will probably go with them. With my older eyes I believe they will allow quicker use in self defense.
A lot to consider but that keeps it all interesting. I got my Shield yesterday and will go shoot tomorrow. Good luck with yours.