Properly done, the cocktail party is a very different kind of gatheration than the traditional house party. Probably the most noticeable difference is that the BYOB tradition simply doesn't work. The reason for this is twofold: first, it is not reasonable to expect your guests to know how to make a decent cocktail, and second, it's a rare (and dare I say typically uninteresting) cocktail that can be made with a single liquor and a single mixer, which is about the maximum you can expect a guest to bring.
So, the first rule of the cocktail party is to provide the liquor yourself. The second rule is a direct consequence of something mentioned above; namely, that it is unreasonable to expect your guests to know how to make a good cocktail. The answer? Make them yourself.
Now, for a good size party (the ones we have here tend to peak at around 30-35 simultaneous guests), this mean a lot of work for the host, though there are things you can do to minimize it (see below). So hosting a cocktail party is not for the faint of heart. But believe me, if you enjoy making drinks, enjoy praise (and if you can follow some of the recipes posted here, or have other favorites that your friends and loved ones rave about, there will be praise...), and enjoy seeing your friends having a great time at your party, then it's well worth it.
The third rule for a cocktail party is to make sure there's plenty of snacks available. Fortunately for me, my wife handles this end of things brilliantly. Fillet mignon canapes, bruschetta, and stuffed mushrooms are amongst her more highly acclaimed additions to the party fare. For those (like me!) who balk at that kind of food preparation, a simple cheese plate with crackers and/or a veggie spread with dip will work wonders. But the basic idea here is to make sure nobody gets too trashed on those great drinks you're making, especially later in the evening.
So, you've decided to host a cocktail party. Now what?
The first thing you'll need to do is decide which drinks you want to serve. It's important to realize that the more drinks you offer up, the more time you'll be spending preparing them. This is definately an area in which I can improve, as I tend to offer up about 20 different drinks, and spend nearly all of my parties working the shaker. If I could bring myself to restrict the offerings to 5 or 6 drinks, I'm sure I could limit the time I spend 'at the bar,' but I haven't managed to curtail it because I'm in love with so many different cocktails.
However many drinks you choose to offer your guests, you'll want to make sure there's a nice balance to suit everyone's palate. Make sure there's a fair representation of strong, sweet, and sour drinks, as well as some lighter drinks (i.e. at least 50% juice or soda). I'd highly recommend having a Collins available for this purpose (my favorites are the Vlad Collins or John Collins I've already mentioned--you can also substitute blended Scotch for the bourbon in the John Collins to make a Sandy Collins). I also think the Espresso Martini is a great addition to any cocktail party, as it gives your guests a great pick-me-up option without having to resort to a Vodka and Red Bull, or something similarly nasty.
Finally, you can add a nice, impressive, touch to your cocktail party by printing up a few menus. Nobody will expect it, it's got a huge "wow" factor, and it saves you from having to explain to each and every guest what drinks are available. Just type something up with interesting fonts (best not to use more than two fonts, though) and send it off to a nearby printshop to be printed on some kind of card stock (something thick). Here's an example of the menu I used at my last party (I folded it lengthwise down the middle, and had 'Cocktails' printed on the front):
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Espresso Martini
Although it only uses a half ounce of espresso, this is a great drink when you need that little extra boost to get you into party-mode.
Stir the following in a mixing glass with ice, and pour into a cocktail glass:
1 1/2 oz. Vodka
1/2 oz. espresso
1/2 oz. Kahlúa
1/2 oz. crème de cacao
Optional:
Garnish with 3 espresso beans
