When you are sending out your birthday invitations
A count of the attendees
Right from the time you send out those birthday invitations to the time when the dinner plates are out, you must make sure that you have an accurate idea of how many would be attending the party. This will help you make the budget and avoid wastage, without falling short at any point of birthday party supplies either.
The supplies for decoration
Simple party supplies like balloons, shining paper, party foam, etc. aren't hard to find. But if you are trying to have a themed birthday party, whether it is mimicking the space world, fantasy world, jungle or a magical world, you have to ensure you have all the supplies handy for the decorations. This could include painted card boards, costumes, capes, hats, shining goblets, lamps, etc which add beauty to the ambience. In fact, even the birthday invitations should be consistent with the themes that you have chosen for the party.
Food is the most important part of any party. You need to take care of the soft drinks, the cookies, the pancakes, the spicy stuff, the starters and appetisers and the main dishes. You have to make sure the birthday food and the delicacies are consistent with the guests who arrive. If it is a kids' party for example, you would want to avoid spicy stuff and have lots of sweets and varieties of ice creams. Similarly you would want to have enough birthday food to last the party and the more the activity, the more variety you would want to have. The season should be kept in mind. If it is winter for example, you might do well to have barbecues and grilled food.
The flavour of the cake is always the choice of the birthday baby, but you would want to ensure that the cake is ready before the party starts to avoid last minute surprises. Getting the candles, cake toppers, wishes written on the cake, etc. should be done in advance. Sometimes you can plan mischief too with items like candles that cannot be blown out, provoking laughter and tiring out the birthday baby.
When you are planning a birthday party make sure that you choose the venue and time properly. Sometimes, your house might not be big enough for the attendees in which case a party hall or even a fast food outlet like a McDonalds' or ports bar if there are lot of young people, can be rented for the day. The time should be perfectly chosen depending on whether it is weekday or weekend and summer or winter.