Rarely does anything in life go exactly according to plan. There are so many variables and uncontrollable ones at that~ like people and the weather. Planning a life event such as a wedding is no exception. The old adage, prevention is worth a pound of cure, seems like sage advice to me.
So, how can you stay prepared?
Stay organized
Use your planner. Keep important contacts, phone numbers, websites, etc. with you. Systematically work through your to-do list and check items off as you do them. Inspiration boards and color palettes will help keep you focused and on track with the look and feel of your wedding. Assign tasks to trusted individuals to help keep you from being overwhelmed.
Do Your Homework/Research
The professionals and vendors you choose should be recommended and researched before you hire them.
Day of Wedding Schedule
Print up a schedule of wedding day timelines and give it to your officiate, attendants, vendors, parents, etc. Everyone will be on board with when and where they are supposed to be. Again, delegate to your trusted individuals wedding day tasks.
Prepare An Emergency Kit
Anticipate not only the needs of yourself and your bridesmaids, but also of the groomsmen and the guests. Items such as a first aid kit, sewing kit, shoe polish, toothbrushes, toothpaste, combs, lip balm, Shout wipes and water bottles are a few of the essentials.
Plan B
Take a look at the scenarios surrounding your wedding day. Anticipate anything that might go awry. For instance, what if it rains? does the hallway or walkway have enough light? what happens if you break a heel? You can also ask your vendors about their "what if" plans?
No worries about things that probably won't happen~ just anticipate the possibility of what might realistically happen, so you can effectively handle any mishaps.

Simply Elegant
Essentially Yours, coordinating wedding invitation ensemble