There are two main things that you will need to put in your "Wedding Planning" notebook: receipts and general information. If you keep track of these two simple things, your wedding planning will go a lot smoother.

Receipts
The best way to make sure you are staying within your budget is to check your receipts against your original amount in your budget. If you go over on something, plan on spending that much less on something else. If you spend less than planned, feel free to use that money for something else (hint: pampering?). Make sure you reconcile your budget after every purchase to make certain that you don't go over budget. You should keep all of your receipt in one place so that you can refer back to them if you have questions about what you got or what the price was.

Information
In the beginning of your planning, you will be gathering a lot of different information about vendors in order to pick the one that would be best for you. You do not necessarily need to keep the information on un-chosen vendors; however, it is critical that you keep information about the vendors that you do choose. Make sure you keep brochures, contracts, and everything in between so that whenever you have a question, you will have the answer at your fingertips. You should have separate folders or binder sections for each vendor so that the contracts and such do not get lost between other papers. Also, if you keep all of the contracts and receipts you will be able to know what you owe without having to make an extra phone call to the vendor.

Once the wedding is over you can choose to just throw away the information, or maybe you can use it for a friend who is getting married and needs advice on vendors. Some people like to keep the brochures to put in a wedding scrap book. No matter what you do after the wedding, just make sure you keep track of everything before the wedding. It's no fun going through a huge stack of receipts after the honeymoon to see how much over (or under) budget you went.