If the wedding includes throwing rice, or an appropriate substitute, this is not the time to use flash. If flash is used, you will just get a bunch of white specks in the picture. You'll get a much more interesting effect by using a shutter speed no faster than 1/2150 of a second. The rice, or whatever, will show up as elongated streaks, and appear more natural. If the location is somewhat dark and your camera offers a flash-and-time mode, you could use this feature. The effect will be even better than straight flash.
At the reception, don't attempt to take a picture that will include the entire wedding party and all of the guests, especially if you want the faces to be recognizable. It is much easier, and more meaningful, to take pictures of small intimate groups of guests. The pictures will be even more meaningful if one or both of the newlyweds happen to be talking and laughing with the guests. Make sure that the face or faces of the honored guests can be seen. Profiles are acceptable, and will often provide the best expressions.
When it comes time for the wedding meal, let everyone eat in peace, but be aware of the moment when the best man or the father of the bride stands up to make a speech and give a toast. If possible, find a location that will allow you to include expressions on the faces of the speechmaker and the wedding party. All the better if the speech contains a few humorous remarks.
Don't forget the little ones at the reception. The younger members of the wedding party as well as the young relatives and friends can be a wonderful source of picture possibilities. Because of the excitement of the day, they will become natural subjects, but try to photograph them early because kids tend to wear out faster than the adults, especially if the reception takes place in the evening, or extends over a long period of time.
When photographing the children, you might want a few posed shots, but your best pictures might be when the kids are just being kids. You'll probably have plenty of chances to catch them on the dance floor or at the punch bowl. Whenever you take their pictures, though, you'll want to photograph them at their eye level. Hopefully, if a child is talking with the bride and/or groom, the adults will bend or sit down so the faces will be at approximately the same level. If not, it would be better to take the picture from the perspective of the adults, otherwise the child will look normal but the grown-ups will appear larger than life.
The newlyweds cutting the cake can be a fun shot, but it can also be a little tricky. Do you want to show their hands as they make the first slice, or would you rather show the expressions on both of their faces, or would you be more interested in the expressions on the faces of the guests? If you go for the faces of the bride and groom with a straight-on shot, you'll also be in position to catch their expressions as they feed each other the first pieces of cake.
When taking the wedding cake shots, be aware of a blank wall behind the newlyweds. This could cause objectionable or unflattering shadows in the background. If possible, shoot at about a forty-five degree angle to the wall.
The bride throwing her bouquet is usually one of the trickiest reception shots, especially if this event takes place on a small dance floor. It's almost impossible to move back far enough so that the bride and all the young girls are included in the viewfinder. If it looks like this might be the case, leave this picture up to the professional.
Be sure to save a few frames of film for the time when the newlyweds leave the reception. This can be an emotional as well as a happy moment.