Use the flash-on mode. This is probably the greatest secret that most every wedding photographer knows. This will expose your background first and highlight your subject. It's great for those scenes in the garden or during outside weddings.
You can also try to use diffused light. Either learn to reflect the flash, or decrease the light with a flash diffuser. You should definitely keep your flash with you, especially when taking shots with a lot of shadows. You need to add some fill flash to the shot.
Try shooting in the RAW. It gives you shots that you can manipulate later.