Once you start planning your wedding, the first thing you should plan is your budget. Do some research and find out how much stuff costs and then weigh this up with how much you can afford and find a happy medium. Stick to this budget as it goes down the line. Don't get yourselves into trouble with money for your wedding, thats not what your wedding should be about. There will always be other things you want at your wedding, but don't if you cant afford it, be realistic.
Once you have decided on a guest list, stick to it. Unless you make some new close friends in the time between making a guest list and your wedding, or some new family members arrive, don't keep adding new people because someone said you had to. Guest list decisions can be the hardest part of planning your wedding, especially if you both have large extended families or if you work with a lot of people, so you have to draw a permanent line under your decisions.
Purchases: Big purchases aren't to be messed with, and once they are bought, don't start second guessing yourself. When you find the perfect dress - stop shopping. Don't start looking around for one that you saw in a magazine or on a blog. Chances are, you bought that dress because you loved you in it. Remember that feeling and be happy with the choices you have made and move onto the next planning stages.
When stresses happen, just remember why you are getting married. You're planning the wedding for you both and for you to enjoy your day with family and friends.