Wedding photos are just like other pictures so the basic rules for photography still apply. Composition and light are important for success, but in this instance there is no alternative to preparation.
The first stage should be to review previous photos you have taken at weddings – which ones worked and which didn’t. It can also help to look at the websites of professional photographers; for example Canon Ambassador Jeff Ascough.
Once you have seen what worked well and what didn’t, it is a good idea to select three to five photos that are essentials and make sure that you capture these on the day. You could also talk with the bride and groom to find out if there are particular photos they would like you to take that the official photographer might miss.
From this you can decide on the right equipment to take. It may be tempting to treat yourself to a new lens or flash. But on a day like a wedding, it is vital that you are confident with your camera.
Make sure that you have a fully charged battery (and a spare) before you set out. The same goes for memory cards; you do not want to find that you cannot take photos because your card is full or is not working properly.