Having a party plan is an important step to ensuring your party runs smoothly. Even a rough plan is better than nothing.
Think about what guests will do on arrival, will someone great them at the door, are there arrows, signage or balloons directing them where you want to go?
How long will it take to play each game or to do each activity? Work out if you have enough entertainment/games. Note: it's always good to have a few extra things ready in cases you need something extra. What's the fall back plan if the weather or some other issue means you need to alter your plans?
What do you do if people bring gifts? Do you have a place to out them, will you open them during the party or after?
If you are serving food you need to know when you need to start heating things up or when to clear a place for people to sit. Make sure you have someone to help you need it too.
Are there speeches, or happy birthday messages to give? Work out when and who will give these? Do they have any necessary equipment? Who's in charge?
What happens at the end of the party, are there party bags to give out, who will see guests leave and thank them for coming?
Thinking through some of these things before the event will help you plan, prepare and enjoy!