I’ll be helping out at a cocktail party for charity next month. Thing is, I’ve never mixed drinks for people from behind a bar before.
So, in your opinion, aside from the basic gin and tonics, bloody marys, what are some essential recipes I should learn?
So, by basic, I assume you mean a highball? Examples of a highball include a gin and tonic, rum and coke, Scotch and soda, etc. Those are all pretty easy, but keep in mind that one very minor ingredient can change the name of the drink. For instance and rum and Coke is just a rum and Coke, but a rum and Coke with a lime is a Cuba Libra.
I guess it depends on whether you’ll be making drinks with the aid of mixes. Making a Bloody Mary with a mix is easy, but it’s an art without one. Same with, for example, a margarita. Or a daquiri. Or a pina colada.
You don’t need to worry about martinis much. Most martini drinkers (I count myself among them) will detail to you in excruciating detail how they would like theirs made.
Most people at a charity function will drink wine, or beer, or simple highballs, I would think. I guess some of the better advice would be to practice opening bottles of wine with a waiter’s corkscrew. It’s an invaluable skill for providing large masses with drinks.
Otherwise, I guess I’d learn to make a Manhattan, a Cosmopolitan, a White Russian, a Long Island Iced tea, and probably a sour (although this falls into the “mix” drink arena).
You are always able to ask people how to make a drink, too. If they can’t tell you, they shouldn’t be allowed to order the drink. 
http://www.webtender.com is your friend. I’d suggest learning a good margarita, white russian, sex on the beach, and cosmopolitan receipe. When I tended bar, at most charity funtions people did seem to drink lightly and more sophisticated beverages, like martinis, scotch and white wine. Or the basic “and” drink…scotch and soda, gin and tonic, rum and coke, whiskey and coke, etc. If you get confused as to whether or not they want gin or vodka (like in a greyhound) ask them “East coast or west coast?” East coast means gin, west is vodka. (Many may ask what you mean then you can explain east/gin, west/vodka. Saves you the trouble of needing to remake drinks, too)
Have fun