I hate hosting parties so I've probably made every mistake out there. Generally at other people's parties, I am amazed at how well they do and good they are at it.

If I had to pick anything though, I'd say that a lot of time the host or hostess is so busy keeping everything in order and tending to everyone that they don't get a chance to enjoy the party themselves. Let people help with small things when they offer so you have time to kick back with everyone later and don't worry about having everything "perfect".

PREPARE! Your better to be over-prepared than under-prepared. I've thrown my fair share of parties, usually with "friends of friends" so I don't always know every person there. Put away everything that could possibly be stolen, broken, or dangerous. We always did a fairly good job of putting away decorations, but at one party we actually had our remote controls for the TV and DVD player stolen. Some people come to a stranger's house, and see it as an opportunity to do whatever they wish. Put anything valuable or breakable in a spare bedroom.
Even if it's just a friendly gathering rather than a party, someone always has too many drinks.
Also invest in some plastic tablecloths, and use them to protect your furniture. I have one too many rings on my dining room table and entertainment stand from a sloshed drink. Take a little time to protect your wood furniture, as well as as electronics. Nothing worse than a glass of wine down the back of your TV because someone tripped.

After a few parties, we went so far as to clear out anything that wasn't end tables, cover what couldn't be moved (60" rear projection tv), and lock up the cutlery. People you don't know can sometimes be tipped rather easily when they have been drinking, and after seeing someone reach for a knife on your counter, you tend to lock them away. A friend had a gathering, heard an explosion from his kitchen, and came in to find that the significant other of one of his friends thought it would be funny to microwave his toaster and run to pretend she was in the bathroom.

Now, you don't have to worry about this stuff if it's only close friends, but it's a good idea to lock up the valuables and breakables anyways. Make sure you have lots of plastic cups, cutlery, and napkins. Provide a method for people to identify their glasses - I always leave out a permanent marker. This will cut down on waste, because many people will get a new cup or plate if they can't identify their own. Having multiple styles of cups, or multiple colors of dishes also help with this. Make sure you have munchies on hand - store brand chips always work well, or cash for pizza and the number put aside.Make sure you have a good amount of ice. Put out extra toilet paper, kleenex, and paper towel. Put out a clearly identified garbage bin - if people see the garbage and it's convenient, they will throw their things out rather than leaving it everywhere.

Then, forget about what's going on. Don't worry about a little spill. Don't worry if anyone needs a drink - as long as it's available, people will be able to find it. Of course, if people ask, provide them help, but don't follow people down enquiring if they are OK - just assume everyone is having a good time. If something gets knocked over, you don't need to rush over (unless something gets broken), just trust them to pick it up on their own. Don't give up your seat at the table - let the late comers stand. Treat yourself like one of the guests.

When it's all over, do yourself a favor, and go to bed. Don't try to clean up. Sleep in. Everything is off the tables so cleaning should be easy - wipe down table cloths, remove, and fold. Pick up disposable dishes (which is much easier than spending an hour washing cups). You'll likely have to give the floor a cleaning, but not too much else. Even after the biggest parties we've thrown, the most we've had to do is pick up bottle caps and cups, throw out tablecloths, sweep, mop, vacuum, and (in one case), clean a drink off the ceiling because someone tripped. Never a whole lot of cleaning to be done, and even garbage was minimal as most people dispose of their own. The times we used disposable dishes were much easier for us. Other than the remote incident, the only other damage we've ever suffered was having a pedestal sink pulled off the wall when a friend decided she'd invite her boyfriend into our bathroom (use your imagination).

But pre-prepare for the worst, and don't worry about anything during the party (unless it's very important and will cause damage), but remember that once It's done, it's done. Remember that no matter how hard you try, no matter how much time or money you put into it, no party is perfect and someone will not have a good time. They'll get over it. If you are having a good time, thats all that matters!

Good luck!

The biggest mistake that a hostess can commit is not showing individual attention to all of the guests. Yes, the food, drink and entertainment are important. Yes, proper planning and coordination insure that things go smoothly; but if you don't show individual attention to your guests, no matter how good everything else is, that's the impression that will remain.

I have attended parties were the host and hostess were so gracious and made everybody felt so comfortable that the ensuing environment was quite festive and enjoyable. Under those circumstances anything lacking in terms of the food or drink are easily forgotten.

I have also attended parties were everything was picture perfect and delicious, but one felt neglected, I always remember those too, and the memories aren't so kind.

What seems clear to me may not be obvious to others, but time after time I see hosts announcing a diner time and then delaying it by half an hour, and hour, two hours, or even more. I have seen people from out of town leave without dinner as they had to drive a long way that evening. I've seen those who are responsible for elderly leave without dinner as the elderly couldn't tolerate long periods of time sitting at a table. I've seen people with all sorts of dietary requirements have to leave and feed themselves on a regular schedule. All of this while the host/hostess go blindly about their way, seemingly oblivious to their responsibility to their guests.

If you invite people to activities and announce a dinner time, serve dinner at that time.