Eat well before-hand. It's generally a long day and there's a lot to get in before the meal.
If you haven't bought gifts on-line, through a gift-service, label your gifts and cards clearly. Upon arrival at the reception, place your gifts on the gift table. There will often be a cardboard post box for cards. All too often, unlabelled gifts are thrust into the arms of the groom, at the church, just before he gets married or into the arms of the bride in their receiving line and there's really nothing they can do with them at those times. Usually someone has to rescue them and remove the cards and presents, so that they can perform their duties. Then later they are unable to thank people, as gifts have not been labelled, which is unsatisfactory for guests and the couple.
Bring some confetti or rose petals . Rose petals are environmentally friendly and some venues do not allow the use of traditional confetti.
Arriving: For most guests the ideal time to arrive at the church or register office is 20 minutes or so before the service begins. If you arrive earlier, before the groom an ushers get there, just wait until they have arrived before entering the church. But do not arrive after the bride. This is a sin and you must do the walk of shame.
Once at the church or civil service location, you must be seated before the bride and (typically) her father arrive. Although it is the ushers job to ensure that everyone is inside and seated in good time, it is often their first time at the job and will be nervous and may not wish to tell people they don't know, what to do. Get inside around 15 – 20 minutes before the service starts.
You may be asked by the ushers whether you are 'bride' or 'groom'. To clear this up once and for all, they have not confused you with the guy in tails or the lady in the white dress. They wish to know whether you are a relative or a friend of the bride or the bridegroom, so they know where to seat you.
Guests with children: It can be stressful taking small children to a wedding. More and more churches have taken this into account and provide a designated play area, with a selection of toys and books, towards the back of the church. The vicar probably won't mention this until you are seated by which time it's too late. Look out for this on arrival and if you are lucky and they have one, head straight for it. Don't battle on in the isles, attempting to maintain silence, until the inevitable explosion occurs, just go for the play area. You'll find quiet play a lot less stressful and no one minds the sound of parents talking to their children and introducing toys to them. They'll just be grateful it's not screaming.
At some point you will most likely be asked to pose in group photos and engage in the tradition of throwing confetti. I'm sure you'll do a grand job.