WikiHow on How to Stop a Wedding

And now you know:

Be very cautious about “crashing” a wedding. If you were not able to contact the bride or groom beforehand, and you are sure that you want to proceed, find out the location of the wedding. Unless you have been invited, you will have to find out where the wedding ceremony is to be held, and the exact time. Ask family members or mutual friends if it doesn’t appear too suspicious, or perhaps check the wedding notices in the local newspaper.

Find a suitable place to wait. Do not allow yourself to be seated by the ushers. This increases the odds that you may be stuck in a bad part of the audience that will make it difficult for you to make your objection. If you have to, wait until everyone has been seated and the wedding is underway before you enter the foyer of the building.

If the wedding is outdoors, keep a good distance until the bride has walked the aisle.
Take charge if things go your way. If he or she decides not to go through with the wedding, it is your duty to immediately escort the bride/groom away from the pressure of their family and friends. There is no doubt that friends and family will be angry or furious and will demand answers if the bride or groom doesn’t immediately flee the scene.

Have a get-away car prepared so that the bride or groom doesn’t have to face the embarrassment of his or her friends and family.