Get started about a month before
You got to plan ahead to have a good party. For a formal party this planning should start about a month before the actual party. You can pull off a very decent casual party if you have about two weeks in your hand. Before you start planning for the party you need to ask yourself why at all are you having the party. Once you have the answer to it you would know what the theme and mood of the party would be.
The very first things you need to settle on are the date, the place and the style of the party. While deciding on the date remember that you can have your party on some other date than the date of the event you are celebrating. It is more about convenience for you and your guests. Try to fix a date which would ensure that most of your guests would be able to come. Regarding the place of the party you can have it in your home or you can hire Catering Halls or Party Rooms. Regarding the style of the party you can have it as an afternoon Sunday brunch or may be a full five course formal dinner party. That depends on the purpose of the party, the kind of guests you are having and the budget you have for the party.
The next thing you got to do is make up a list of the people you are going to invite and send them the invitations. Get your invites printed for the formal parties. You can try your hand in doing the invites yourself for a casual party. Keep a checklist with you of the guests you have invited.
Plan the menu for the party. Dont try and experiment too much. And if you have to, then first try it out yourself before settling on the final menu.
Decide on the decorations as well and the seating arrangements. Complete the shopping with enough time in hand. Look out for sales going on where you can get a bargain. Also make sure you will have the things on the day that you plan to borrow from your family and friends.
Make a budget for the party and stick to it. Keep track of the overall expenses and dont go beyond what your budget permits.
Do the groundwork a week before
Having received the RSVPs make sure the status of all the invitees from whom you havent heard. Call them and find out if they are coming or not. This would help you have a clearer picture of the actual food and space requirements.
Have your housecleaning done and check if all the appliances you will be using for the party are working properly.
Check your party list and grocery list and shop for everything but the most perishables. Dont forget about the little things like matches and candles.
Order anything that you will need to have on the party day from flowers to meat to seafood.
Decide on the music for the party. Good music sets the mood for the party without being too intrusive. Also have a camera ready to capture the precious moments. Make sure you have the film rolls/cassettes and the batteries. If you are using a digital camera check if you have enough space to store the images.
The last lap one to two days ahead
Arrange for a first aid kit. You never know when you need them.
Make yourself a timetable as to when you will be cooking on the day. Get some work done beforehand by preparing some non-perishable items like dips and snack mixes.
Shop for the most perishable items.
D-day an hour or two ahead
Have all the items on the menu ready.
Get your table ready and make sure your guests have an easily accessible garbage where they can put down their dishes once they finish.
Before the guests start pouring in have the music playing and the candles lit.