Watch this video of a US Marshall and his accidental (negligent) discharge. Then read what I have to say about preventing this from ever happening!
You want to use a holster that allows you to take your gun off and on WITHOUT unholstering! This is the most dangerous part of carrying concealed – drawing and reholstering! I recommend an IWB holster with one clip — not two clips — so that you can easily remove it and replace it with the gun still in the holster. If the gun never leaves the holster, you can’t accidentally pull the trigger!