1. Emulate charismatic behaviors.

2. Create intentional social space for those around you.

#1 is what I would call "politeness" or "charm". There's a strictly traditional (conservative/Victorian, though I'm coming from a western perspective) way to do it, but every branch of counterculture has effectively created it's own version. I would count everything from chivalry to fashion here: anything that helps people make useful assumptions about you. As soon as you are having a comfortable conversation with someone, you are ready for #2.

#2 is more meaningful and more genuine, but it's also really difficult to do without starting with #1. Every person in every social interaction exists somewhere on a spectrum from anxious to confident. You can't directly change a person's position on the spectrum, but you can move the spectrum itself.

Lately, I've been watching the sketch comedy series, I Think You Should Leave with Tim Robinson. Robinson is a master at playing with that spectrum: He squeezes it together, making intensely awkward cringe moments for the characters; then at the most unexpected moment, he stretches the spectrum out with impossibly intimate social interactions where everyone in the scene is suddenly loving and respectful to each other, totally leaving behind the awkwardness they started with.

Anyway, the social game is to just filter noise and find the signal you want.

When you look at all the self help books on this, they're all very scholastic and merely teach techniques that sour once they're well known. A famous one is repeating someone's name back to them. I can't think of a smellier tactic than that.

The goal of any social interaction is up to you. You don't have to be best friends with everyone. That's just a bonus when it happens. I suspect this is where the emotional drain occurs for a lot of people. You can't make people like you, but they're more likely to do so if you can make them think twice about disliking you.

Determine what appeals to someone, adjust your style accordingly, funnel them towards what you really want to talk about, make a decision about how strong the signal is and whether to move on.

I hope you see how this is kinda like gambling. This is how it can be very easy for someone to talk to 50 people and still be super pumped up to keep going. The upshot is that energy really attracts people. This energy... that's the charisma. You express it your own way and that's it. Keeping it going shouldn't be hard for anyone unless they had a murky goal to start with.

I'm trying to imagine how I would feel if I were in your shoes trying to woo customers. I think the negative feelings I would have afterwards would be forms of anxiety: perhaps I'm worried I didn't present myself well, I'm thinking about a blunder I made, I have to recover because there was one guy giving me a hard time and making it difficult for me to be "on", or maybe I did everything right but something else upset me (I don't like the customers, what I'm selling, the venue...).

From what I gather you would feel none of these things.

Edit: Another thought: would you relate being at this kind of event to performing a concert? I usually am nervous, but eventually my instincts kick in and I can play. Afterwards my first thought isn't "let's do another," it's more like "I want some ice cream and to sit down." And I admit I kind of feel this way with big social events like the one you describe - maybe I'm focusing too much of a more negative feeling when I think about what you mean by being drained.