I go to a lot of cons. A lot. In fact, if there is a con in Socal, I'm usually there. If there's a con in Norcal, I'm probably there.
I love cons. We try to do 30+ a year in all sorts of genres. I walk the aisles at every one trying to find new stuff, and am constantly met with artists and vendors that shy away from talking to me, or engage me with their eyes low and voice squeaky. People's con game is weak, son.
Engaging at shows is so important to get fans and make more money. Fans want to meet you. They want to buy things that excites them. They want to find a cool indie person and say "look at this! They are gonna blow up!"
You are the product as much as what you are selling. They want to go back to their friends with a story. They want to hang your art on their wall and remember that cool person they met at that awesome con.
With that being said, I know not everybody is as outgoing as me, so here are a couple of tips that can help you get you talking to more people.
1. Smile and say Hello - I know it's hard, but the simple act of smiling makes your more approachable and sends endorphins to your brain to help you get over the con mopies. If you are ever down at a con, just smile like an idiot for 10 seconds and you can force your mood to improve. Once you are smiling, just wait until somebody looks your way and say "Hello". The advanced move is saying "Want to see something cool?" If you can get to that point, you'll get tons more foot traffic, and with more foot traffic, more people will buy your stuff.
2. Eat - Everybody gets hangry when they don't eat, and most of us eat terribly at cons. Spending a few minutes at a supermarket getting grapes, water, and some pre-made sandwiches will do you a world of good. Candy and sugar is going to make you drop like a rock and make you harder to be around.
3. Focus on your best selling product(s) - Most people get overwhelm at booths, including the vendors. If you know which product sells the best, then just ask if they like that product, and then you can talk about it. If they say NO, then you can ask what they DO like and it's easier to engage with them.
4. Be a fan - We are probably the biggest fans around. I mean we love comics, or art, or books, or photography so much that we created something. We are the Super Duper Fans. So you can geek out with people about what you love. Talk to them about other books and why you made your project. Talk to them about what it means to you, and why you exhibit. Be you.
5. Let people feel your product - If you can, put a product in somebody's hand. People are many times more likely to buy if they are holding a product, feel it, and can imagine owning it, than if they are just looking at it. If you do art than frame it and show somebody what it would look like. If you write books then let them hold it and flip through it.
6. Finally, remember that people want to buy - People are there with money to spend. Remember that. At a big con they have probably been saving for months. They want to buy stuff. They've thought about what kinds of stuff they want. It's up to you to show them your stuff is their kind of cool.
You aren't going to sell everybody. In fact you aren't going to sell almost anybody, especially at first. However, if you can get comfortable selling then you'll have a better time at a cons and you'll even leave with some money in your pocket.
Just don't forget this. If somebody BUYS from you, make sure to get their email and give them ways for them to follow you!