Here is what i do, and it seems to work.
When im taking shots, especially formals, I need all eyes on me. We always have parents and friends standing behind us of off to the sides with the P&S in hands. Some even have nice DSLRs.
After i get my light set and im ready to shoot, I turn and tell everyone present the following
"Hey everyone. I need to have the bridal party looking at me for these shots. So, Give me a couple of minutes to get the shots they have hired me to take and I will step back and you can get the shots you want. Oh and all you nikon shooters have to stand in the back row".
People laugh and Ive have never had a situation where the guests did not comply with my requests. And i get my shots for the family, come across as reasonable and a little funny.
It works, i dont have point and shoots triggering my lights, takes about 1 minute extra time and everyone is happy. Its also a good time to tell them I will sell them a photograph " super cheap" and hand them a business card.
I have no intent or desire to sell them a photo. But it does allow you to gain some control in a manner that comes across as reasonable and non threatening . And ive have had people contact me and buy some prints which i sell " super cheap". And, most importantly, ive also booked a couple of weddings with other family members because i dont come across as a photographer who is there to grind everyone for prints.
Look this is going to happen at a wedding so you better get ready and be able to handle it. I could care less about selling anyone a print. But I do care about getting my job done, and so does the family that hired me. It works and I have minimul problems.
Take a negative and turn it into an positive. Ive seen many photographers do some really stupid stuff when it comes to this issue and I believe the way they handle it has hurt their reputations. You need to be flexible. Its really not that big of a deal and to be perfectly honest, when it happens, the mothers of the brides usually step up and say something to the intruding family member. It stops instantly when Mama Bear says something.
I had one mother tell a family member (she jumped in twice and triggered my lights both times) " Im going to shove that camera up your a$$ if you do that again". Everyone laughed, she apologized and we moved on and had a great time. Swear to god, it was a priceless moment, but it could have been a negative if i handled it wrong. Moms are awesome so make them your very best friends when photographing at a wedding.
Hmmmmmmmmm. 
