First and foremost, remember that the "business" part takes precedence over the "pleasure" elements; your priority should be promoting your professional self, not being first in line for the buffet or bar. That might look like turning down an offered appetizer when you're in the middle of a conversation or waiting to refill your drink until a good time to pause comes. If you abandon a rolling conversation to make a trip to the cheese platter, you may return to find that it has moved on without you, and that you've lost your stake in it to someone else.
Likewise, try to keep at least one hand free at all times. Instead of holding both a drink and a plate, choose one at a time. Wear a cross-body bag (like a simple purse or a messenger bag) instead of carrying your phone or a clutch in your hands. This allows you to shake hands, take items offered, or gesture as you're talking without pointing a shrimp at someone while you do it.
When eating, keep portions small and don't overcrowd your plate; you can always come back later for more (or stop at Chick-fil-A on the way home). Finger foods that aren't going to drip sauce, dressing, or cheese down your hands or shirt and don't require a fork are the best bets.
Finally, when it comes to alcohol, you should never try to drink if you're under 21 (even if you think you can get away with it, someone is going to notice, and that's all anyone will remember about you). If you are of age, two drinks is the limit—and that's total, not two beers AND two glasses of wine. Stick with two drinks max, and you'll always be ready to put your best self forward to whoever comes your way!