I usually start the initial planning 3 to 4 months before the big day. This is because I have learnt over the years that if you want a specific venue or entertainment option, you need to book way ahead of time.
This also allows you enough time to do your research (see below) and get some of the prep done, so that you are not overwhelmed with tasks in the week leading up to the party (things like printing name cards or activity sheets, putting together the party bags, gathering all the bits and pieces for the decor, and so on).
Our good friends Google and Pinterest are great places to start, even if you don't have a specific theme in mind. A bit of searching will inspire you with a vast array of options for themes - just try searching on "party ideas for boys" or "teen girl party ideas" etc. Once you have decided on a theme, you can then find ideas for related activities, decor and food, as well as where to buy all the goodies. This will save you heaps of time as well as money, as you can shop around online for the best options.
Draw up a timeline
Write a list of everything you need to do in terms of preparations, and then group them into time categories like 'On the day', '1 day before', '1 week before', '1 month before', etc. As you think of more things to do, add them to the relevant time categories - this will help you see how much you will be trying to do, and hopefully help you avoid rushing around in a last-minute panic. Do as much as you can as early as you can.
What I did
My son wanted a racing themed party at Kew Traffic School, so after we managed to get a booking (which was more difficult than I expected) I started researching on Pinterest for food and decor ideas. I then searched online for the supplies I wanted, and had plenty of time to either order online or go to the shops to get it all.
The invitations went out 2 months before the date (I would usually send them around 4 to 5 weeks before the date, but since we could only get a booking on the public holiday I wanted to give people extra time). In the months leading up to the party I slowly gathered all the decor, designed and printed the name tags and party bags, and bought all the non-perishable food and drinks. Two weeks before I filled all the party bags, two days before I bought the rest of the food, which meant the day before the party all I had to do was the baking. On the day I had minimal final food preparation to do, and everything went very smoothly as we were well prepared and got set up at the venue in just 5 minutes.
As Winnie the Pooh says "Organizing is what you do before you do something, so that when you do it, it is not all mixed up". I think that says it all!
If you would like some extra help with planning your child's next birthday party, then how about winning a copy of "The Best-Ever Party Book"? I have one copy to give away, and all you have to do to enter is comment below (by 13 Nov '13) and tell me about your party planning blunders!