1. Articulate your vision
The bride and groom should each begin by writing at least one full page of their "dream" wedding. This will help them to see what is and is not important for the day. Go wild with ideas and sort through the reality of those ideas as you plan.
2. Manage your budget
Be clear about your full budget -- if others are participating in the funds, know what they are willing to give in the beginning so that you can plan around the figures. Sometimes a bride or groom will say that his mom and dad will pitch in but they are unclear how much.
3. Research vendors
If the bride and groom buy a "package" from a venue, be sure to check with the photographer, florist and musicians etc. to find out that the quality and kind of service is what you would choose on your own. I just heard a bride say that she bought a package in Hawaii and now doesn't like any of the photos. Do your homework as if you were not buying a package and, when you check, make a change if you are not happy with their recommendation.
4. Make it your own
Go ahead and personalize your wedding -- one bride and groom I worked with named the tables after events in their relationship -- i.e. First Xmas, trip to Hawaii. A photo of that event served as a table locator instead of a number, Vendors see lots of great ideas and may be happy to offer them to you if you ask.
5. Get the price right
Read all contracts carefully. Make sure that you understand what you have agreed to. Know ahead if the band will stay later if you and your friends want to dance longer, and know what the price will be. You don't want a several hundred-dollar surprise.
6. May I have that in writing?
If you make any change in your contracts before the event, ask for a confirmation in writing. If they are not willing to do so, you write it up and snail or email to the vendor. Make sure that they receive it and agree. If you have decided to change the floral colors to periwinkle and rose because the attendant's dress color changes, you want to know that the florist got the change.
7. Keep the peace
Communicate with all members of your families what events will take place, at what time, locations and attire if necessary. People like to know in advance what is planned. I remember a stepmom who was expected to be at two semi-formal events and knew very few details. She was asking me since there was poor communication and tension in the family dynamics. Deal up front and early on.
8. Know your time
Be sure you understand what time your event ends. Sometimes a venue rents the room until midnight, but really needs to start clean-up at 11 pm, meaning that is the time your reception is over. Other venues mean that you can end the event at midnight, and they clean up after time. Be clear about the policy.
9. Hold the mop, please
Ask the facility not to begin clean up until the time agreed upon. Nothing will clear a room out faster than the wait staff busing the tables as guests are visiting or removing everything on the table as people are dancing, giving guests the idea that it is time to leave. I have heard couples lament that people left "so early."
10. Confirm, confirm, confirm
A few weeks before the wedding, confirm in writing with all vendors, especially noting any changes, no matter how small. Ask them to sign and fax back (or email) their agreement. At a recent wedding that I coordinated I asked the other vendors what was the worst wedding mistake they had seen, and two of them said a caterer that did not show up. Mistakes in scheduling can happen, even with the best organizations. I believe that this final confirming should be in writing.
11. Create a "Day of" roadmap
Do some research and find out how to make a timeline -- it enables a family member or guest to know what to expect that day and call for help if something does not happen as it is planned. Someone who is helping on the day can know what all plans are and supervise arrivals, questions etc. Mine is about 10 pages long and comprises every detail of THE day.
12. Consider a coordinator
Educate yourself as to how a professional wedding coordinator can help you with the wrap-up (and managing the day) of your wedding. Thousands of dollars are spent on a few hours and you want everything to go as you planned and paid for. A professional is working the wedding day so that the couple and families can relax and enjoy. It is worth the reduction of stress and is generally much lower in price that a couple thinks.