What lenses are you using?You can always fill flash or better if you have speedlights. Try aperture priority the switch to manual. For the reception you can also use the natural light coming in from the window. Depends on your composition.

My first tip for anyone thinking of photographing a wedding would be to have a run through before the day and get comfortable with your camera and flash. The inbuilt flash on most digital SLR's isn't going to be sufficient to light a group of people in a large space nicely, it's much better to get a dedicated flash gun.

I use aperture priority for most of my photography. It's sometimes nice to use a wide aperture that puts the distracting background out of focus. Other times, a small aperture is required to keep everything in focus. You always have to make sure the shutter speed is sufficient to freeze movement and stop camera shake. Use flash, a tripod or higher ISO if you see any image blur. I quickly check every photo after I take it on the LCD screen. I also zoom in on the preview with really important photos, to make sure they aren't blurred.

When using the flash, I always make sure the shutter speed isn't too fast. Every camera has a maximum shutter speed to use with flash or there is cut off and that ruins the photo.

Perhaps the best tip is to let a professional take the photos. It's a really tough job and if anything goes wrong, there's no way to do it again.