1. Pick a theme
I always find party planning a hundred times easier once I have a theme to work with, it makes party ware buying so much easier to do. Even when it's as simple as a coloured theme rather than a character or specific activity like a crafty party. Having the specific theme in mind will help you stick to what it is you have in mind rather than straying onto things you won't end up using and just picked up because.
2. Buy in advance
Grabbing things as you go even if it's months ahead of your planned party date. I usually start at least 3 months early but even tho I start that early I usually end up forgetting all sorts but it's usually only the little things like blu tac, sellotape etc all things I can run out and get the day before.
I love writing lists! So with a birthday party it's the perfect excuse to write another list of children invited so I can work out numbers for party bags, party game prizes etc. I usually have some spares as you just never know how a party pans out you could have siblings turn up too or you could have one or two not turn up. This is also handy for food planning there's nothing worse than being short of food or even over prepared with food and having to bin so much.
4. Book in advance
Again I'm usually a good few months ahead when it comes to booking halls, bouncy castles etc if I plan to have any of those. This year will be Daisie's first hall party and although her birthday isn't until July a hall I planned to book was fully booked other than one slot on a date after her birthday even than it was only free for 2 hours before someone else would be arriving for their booking time. Which didn't leave us a lot of time to get in set up and cleared up and out. Which has ended up being the same turn out for a different hall that I've booked with, It's always best to book well ahead to save any disappointment which will also help you work out final costs for the day.