Try setting the camera for rear curtain sync. This allows the flash to pop at the end of the exposure.
What this does it let in some of the natural light from the hall, church or where ever you are, then pops the flash at the end of the shot. It makes a big differance with flash photography. Be careful as if there is movement, then sometimes it shows the movement in the image. It will work better for posed shots where no one is going to be moving during the shot.
Also, get the book by Scott Kelby which has an entire section about wedding photography and is where I learned this trick.http://www.amazon.com/Digital-Photograp ... 737&sr=8-1
Best book on the market for under $20!!