Whoever said "You can be too prepared" didn't know what the heck he was talking about. He certainly had never planned a wedding!
The one mistake I see brides making time and time again is leaving things to the last minute. A lot of people think that it is somehow inappropriate to start looking at things too soon after getting engaged. NOT TRUE! Also, going out to buy a dress before picking your venue and date is crass. EH-EH! WRONG AGAIN! The most imporant rule of planning is: You can never start planning too soon.
Whilst there are perhaps technical implications to some of this - sometimes it is more prudent to wait until you have other details fixed before buying your dress - what's to stop you from building your big day around the dress. Why not? Planning is about shaping the day the way you want.
Of course, it is sensible to prioristise what is important to you on your big day. You also should consider what is going to take up the most of your budget. Venue hire and catering usually eat up the biggest portion of the money.
So first and foremost after getting engaged, you must set the budget. Make it realistic. Whilst it is fair to factor to factor in contributions from family whom you know for sure will want to contribute, it is unrealistic to try and factor in future savings (i.e. money you don't already have in the bank.) However, agreeing not to start planning a wedding until you've got the amount you want to spend on the wedding saved up is a sensible idea.
Weddings should be paid for 100% from savings. I do not avocate taking out loans for weddings as the interest you will end up paying on that loan will render the wedding 100% not worth it! Small items that you can use again after the wedding, such as shoes or wedding night lingerie, could be paid for from your day to day bank account, but only after all your monthly expenses such as your phone bills, other utility bills, insurance payments, petrol/fuel costs, food shopping etc. have been covered. Also, never put wedding payments onto a credit card unless you know you have the money in savings to clear it.
It may help you to set up a seperate bank account for all wedding expenditure. This will help you to see exactly how much is being spent on what. If you cannot do this, make sure you keep a spreadsheet detailing all this information. A spreadsheet can also help you keep an eye on any funds that are due in the future.
Although it may well be the single most important day of your life, your wedding is not worth getting into debt for. Be smart with your money.