In one word: binder. It does not have to be fancy, pretty or designer. It does have to be sturdy and high capacity! You can find pre-made planners, but they are generic and you'll end up changing things around anyway to suit your specific needs. So just go to your office supply store and pick up a heavy duty 3” binder, dividers, and a large pack of good quality sheet protectors.
A good place to start is by creating label tabs reading:
budget, payment tracker, guest list, equipment rentals, floor plan, flowers, photographer, groomsmen, bridesmaids, hair and make-up, cake, rings, stationary, honeymoon, gifts/thank you notes and favors. These are the most common, but you can then add or remove tabs as you need.
Keep track of all of those inspirational pictures and articles about flowers, hair and make-up, dresses, and invitations which you’ve already printed out from web sites or ripped out from magazines. Hole-punch them and put them in the appropriate section. That way when you visit with vendors, you can whip the picture/article out and get your vision across clearly.
As you buy things for your wedding keep the receipts in the relevant page protectors for each section, too. This will help you both find receipts in case of returns and track budgeting. If keeping receipts in different sections doesn't suit your needs, keep them together in a single plastic pouch; I like ones that go right into binders that have a zip-lock closure.
Same for contracts with vendors: Make sure the contracts are all visible, placing them in page protectors, too. File them away in their specific section. Keep all correspondence from vendors, or take notes when in communication with them, especially when talking prices. They might say they'll give you some kind of price break but may not remember when the time comes to sign the contract.
Keep all response cards from guests so that you can triple check your guest list.
Keep track of appointments and dates with your calendar pages.
You should carry this wedding bible with you at all times. You never know when you might need it or when a vendor might call to change times of appointments or ask for information.
On your wedding day, make sure to hand your binder over to you "wedding planner for the day". She will be able to use your systematic binder to help understand exactly what payments are to be made to vendors, tips given to the appropriate people and that everything from flower and cake placement to wedding ceremony line up to your schedule for the day can go according to plan.