The golden rule for photographing a wedding is plan, plan, plan. A photographer who turns up at the venue having never previously visited it, and without any knowledge of the light or local environment, is certainly less likely to deliver excellent images.
Talk to the couple well before the wedding and get a clear mental picture of what is required. Get a list of any special photographic requirements and make sure it is available on the wedding day. Then visit the venues, preferably at the time of day the ceremony is to take place, and observe the light and local conditions. It is also well worth scouting around for the best locations in which groups, couples and individuals might be photographed. Also, don't forget to create an inclement weather plan. Where would the bride and groom be photographed if it poured with rain on the wedding day?
Having obtained an overview of the wedding locations and photographic requirements, it should be possible to plan what equipment to take on the day. It is best not to be too cluttered with every lens and gizmo, but it is even more important to have available everything needed for the occasion.
It is worthwhile arranging a brief meeting with the couple a day or two before the wedding. This provides an opportunity to hear the final plans and get up to date with any changes. It is also worth asking whether any surprises are planned, such as the release of doves or balloons. Such events require prior knowledge to avoid being caught in the wrong place with an inappropriate lens.