In preparing for my own wedding (well, both of them--the civil ceremony and the religious convalidation), I had one major thing going for me: the insight, gleaned from extensive liturgical preparation, that after all the preparation is done, the ritual itself can't go "wrong." No matter how the actual event compares to the preparation that came before it, "going as planned" isn't what makes a ritual successful. What makes ritual successful, beautiful, and memorable? The loving participation, intention, and presence of each person involved.
If I could impart a piece of advice for each couple I work with, it would be this: approach your wedding preparation the way you want to approach your wedding day, and approach your wedding day the way you want to approach your marriage. If you use the love you have for your future spouse and your family and friends as the focus of your preparation, your wedding will be perfect--no matter how many details go astray.
The other related piece of advice I'd throw out there for couples, even though decisions like this are already made by the time a couple meets me, is to choose a wedding party based on who will give you unconditional support both 1) throughout the wedding preparation process and 2) on the Big Day.
Obviously unconditional support depends on a couple's willingness to meet their friends and family halfway--if you're in the habit of acting like a meanie or a bonehead, that unconditional support may rightly waver. But I'd suggest--and perhaps this is radical of me--that it's not necessarily the folks who are supposed to be closest to you that will make the best wedding party members. "Supposed to" won't necessarily cut it when it comes to this monumental day in your life. Ask yourselves: who among your family and friends has a history of going the extra mile to help you in both big and small things? Who among your family and friends has a history of treating you like dirt, betraying your trust, acting passive-aggressively, lashing out, or abandoning you in some way?
Folks you can rely on when the rubber meets the road are the ones you should ask to be in the wedding party, even if you don't have as much history with them as you do with others. Why? Because they'll be standing strong and smiling at you--and will really mean their smiles!--when the stresses are highest. They'll remember that the Big Day is ultimately a day to celebrate you and your beloved. And then, rather than harboring regrets or hard feelings toward one or more members of your wedding party after it's all over, you'll be able to thank them again and again for the wonderful memories they helped you create.
If you have any stories of those who are going / who went the extra mile to make your wedding day memories amazing, share them here!