Keep your flash above the lens for vertical shots as well as horizontal, even if you have to buy an off-camera extension cord and a Stroboframe bracket. Nothing says"amateur" like strong shadows to the side of everyone because the photographer turned the camera (and shoe-mounted flash) sideways for vertical shots.
Shoot with a slight wide angle lens, not a "normal," for all but the close-up portraits. For medium format, that would be a 55mm to 65mm lens; for 35mm film or full-frame digital, that is a 35mm lens; for digital cameras with a 1.5X factor, use a 24mm. Using a slight wide angle provides two benefits: (1) increased depth of field (and, therefore, decreased chance of out-of-focus subjects), and (2) increased battery life for your flash because your will be closer to the subjects (important when photographing the entire wedding party) and your flash won't have to put out as much light. You will not have any noticeable distortion when using a slight wide angle.
Use a good bounce diffuser, preferably one that has a solid bounce surface, on your fash. Don't shoot direct unless you are shooting a large group and you need the power from a distance. I recently attended a wedding where the photographers were using the Lumiquest bounce attachment - the 80/20 one with the "holes" in the bounce surface so 20% of the light goes forward and 80% was supposed to bounce off the ceiling. BUT ... they were shooting in a cavernous 3,000 seat church with a two-story sanctuary and black-painted overhead (girders, HVAC ductwork, catwalks, etc.) with NO chance of any bounce. They also use shoe-mounted flashes; no off-camera flash for vertical shots. I cringe thinking about how those images turned out.
Use the lowest ISO (e.g., 100) for all flash shots and any shots taken outside during daylight. Save the high ISO settings for available light shots during the ceremony.
Don't shoot just anything. Every image you take should be one that someone will want to buy a copy of. (For example, don't take endless shots of the backs of people's heads in the receiving line.) That is how I trained myself to think when I shot film. In the days of film, if I shot 100 images and only sold from 30 of those, then the cost of the remaining 70 images ate into my profit.
Remind the bride and groom (and their parents) that you don't know the people at the wedding and reception and you have no idea as to who the special guests are that you must get shots of. Have them help you get the shots they want. I once photographed five generations of women in one family at a wedding (great-grandmother, grandmother, mother, daughter, and granddaughter). I sold more prints of that one shot than any wedding image I've ever taken. However, I would not have known to group and photograph those five people unless I had been informed.
Get there an hour before the wedding and plan on staying until after the bride and groom drive away.
I could go on, and Dynamike already provided a good start to this thread. I imagine you will receive a wealth of additional comments. Happy shooting, and please post a few.