First of all, forget the food and the decorations. If you're thinking about food and decorations, I can't help you. It's not that they don't matter, it's that they don't matter very much. And that, if you get everything else right, the food and the decorations will take care of themselves. And if you get everything else wrong, the food and the decorations won't salvage it. People planning parties seem to get hung up on food and decorations, as if that's what it's all about. Martha Stewart and her ilk perpetuate this. But that's the first lesson. Forget about the food and the decorations entirely.
What matters? The people. That's what's going to make a good party. Interesting people who are having fun and interacting in a relaxed way, that's a good party. Who is coming? Who do you want to have there? And what do you want to have happen? I like to throw parties where people will meet people and walk away with at least one new friend, one conversation they didn't expect to have, one potential new romantic interest. Are your guests interesting? Do you have enough different kinds of people coming, so that they'll have something they don't experience every day? Are they going to be in the frame of mind to appreciate one another?
So the questions you've got to think about are who's coming, who CAN'T come if some people come (e.g. ex-boyfriends or girlfriends, or sworn enemies. This is rare but must be considered from time to time), and what their social energies are. There are people who are lively minglers, there are people who will shadow the host or hostess, there are people who will find the people they already know and clump together in a secure pack, having conversations they won't remember. Most people can be any of the above, in different social settings.
You've got to think about how to set the party up so that each of these sets of people feels supported and at ease. You've got to break the ice, give people a reason to mix and something to talk about that doesn't put the burden on them and their small talk skills. Alcohol helps, of course, but at any party the beginning is when people who don't know one another are nervous and shy, and so if alcohol is a factor at all it won't be helpful during this time. These are hostess tricks -- if you do them well, they seem invisible to the attendees.
The things that work best at my parties are projects. At a big Thanksgiving dinner for "orphans" last year, I assigned two shy outsiders the task of making placecards. I gave them glue and scissors and glitter and markers and funky paper. I told them to go around and find out people's names and make up namecards for the table. The resulting placecards were funky and hilarious; the two shy people had an easy subject of conversation, and things began to flow. Then I had the people who knew the least number of guests do the seating arrangements, so that people were seated serendipitously around the table. At a bridal shower I threw I did something similar -- there was a craft table and the guests all were asked to make a thank-you card that the bride could use to send a thank you note for a shower gift. It wasn't a formal activity, where everyone did it at once, but people moved around in twos and threes and chatted and drew and colored and exclaimed over one another's cards. At a summer party I asked a couple of guests to get flowers -- I pulled down a bunch of vases and sent them out into the garden with scissors, giving them permission to cut anything that looked pretty. I asked some of the others to help me set up the table -- one long table for fifteen people cobbled together out of many shorter ones. If you assign these projects to groups of guests they bond, they feel useful, they laugh and joke, and they do a great job. The flowers and table set-up at that party were more beautiful than anything I could have done. (This is why I say not to worry so much about the decorations. Your guests can help you with the necessary decorations.)
At a Christmas party I had guests make ornaments to hang on my tree. Some didn't bother, but some found a haven in the project corner, and found a way to talk to people that they might not have done. At another summer party I had a huge bowl of temporary tattoos, and I went around the party grabbing people and telling them to choose tattoos for one another. That was one of the most successful parties I've had. Once in a while I break out wigs and send them around to people, or I bring out hats and tell everyone to take one. I routinely put people in charge of the grill for a time, or I send people off to go count the number of people who are drinking wine and make sure their glasses are full. I find that folks are happy to do this. They feel like they're helping me (which they are), they get to meet one another, and there's no pressure to be perfect. People relax when they see that the party is in everyone's hands.
When I began throwing parties I tried to do everything myself, and I worried about the food and the decorations. What I learned was that this was totally unsatisfying, for everyone. I wasn't relaxed, and didn't have time to attend to my guests. The nervous guests, the host-clingers, were crowding me in the kitchen, volunteering to help, while I was wishing for them to be out of my hair, feeling apologetic that I couldn't entertain them, and worrying that they weren't having fun or talking to the other guests. I learned that asking them to help me, with certain sociable activities, gave them a sense of purpose and helpfulness, and let me relax and mix a little bit more myself.
At this point, my parties tend to go well. People know that when they come to my house they're going to have fun, and it won't be fussy. People show up with mistletoe or marshmallows and chocolate bars and graham crackers or they throw down a piece of cardboard in the garage and announce a breakdancing contest. They bring musical instruments and suggest a jam. If it's a dinner party, I get innumerable offers to help, or to bring things, and it turns out like the Stone Soup fairy tale -- from my stone, the contributions of my guests create a feast. Sometimes, if I really feel like cooking, I'll throw a bona fide dinner party that is about the food, but in those cases my guests bring flowers and wine and so I only worry about timing and oven temperature.
People, my friends. The key to a good party is inviting great people, and asking them to help.