I can't stand high-pressure salespeople. The most memorable one was a guy from Home Depot who was selling water filtration systems. I was willing to listen to his spiel but he spent 45 minutes trying to convince me that buying his system would save me hundreds of dollars a year on SOAP so the system would pay for itself after 5 years. When I itemized our annual expenses for dish soap, laundry detergent, hand soap, shampoo, etc., and came up with less than $50, he remained polite as he packed up his stuff, but you could tell he was absolutely furious because he kept cutting off his assistant who was suggesting a different demo. Interestingly, the demo he was suggesting was something I had asked about early on in the conversation but the first guy said no.
The best salespeople ask the right questions to qualify a potential customer so they don't end up wasting time on people who are just window shopping. Like a couple others said, try to engage them and get to know them rather than just asking questions they can easily dismiss. To be honest, I think the "How can I help you?" and other generic questions are too cliche, and if that's the first question I get, I almost always dismiss it with, "I'm just looking," because usually I haven't had a chance to look at anything yet. That pretty much shuts down laid-pack salespeople and reveals high-pressure salespeople, from whom I pretty much never buy anything.
Grumpymike has the right idea about finding a good icebreaker. If you can't think of anything witty, a decent fallback is, "Let me know if you have any questions about anything." It doesn't give your visitor a chance to shut you down, it's a better greeting than simply, "Hi, how are you today?" and it leaves room for you to follow up later without seeming like a pest. You can also use it if you're already engaged with someone else.
After they've looked a little at your stuff, you can ask if they have any interesting pieces already, and if so, follow up with more questions about those pieces to find out what they like. Ask if they can show you some pictures of a few things they like. If they seem to be looking at something in particular in your booth or portfolio, ask what drew their eye to it and what they like best about it.
There are also a lot guides to types of customers and how to sell to each one, such as this one, which might be helpful:
http://www.businessknowhow.com/marketing/personalities.htm . These might or might not fit your sales style, but may still be helpful in qualifying potential sales.