A good holster must be made for your pistol. Generic, one-size-fits-many models don't do a good job of retaining your pistol and are generally not very comfortable.
A good holster should be molded tight enough to retain your pistol without straps or tension screws, and it should cover the trigger guard. It should also have a reinforced mouth that keeps the empty holster open for easy one-handed re-holstering. The holster's belt loops or slots must match your belt so the holster stays put for all-day comfort and a sure, speedy draw.
Shoulder holsters work well if you spend a lot of time seated in an office or vehicle, and they're nice for outdoor carry. In fact, they really come into their own for those who hunt with the big revolvers because of how they distribute weight. For outdoor use, comfort and security are paramount since drawing the gun is not really an issue. Those who use the shoulder rig for concealed carry need to consider how the holster positions the gun on the body, as well as proper retention.