Read the person's name card and try to understand how to pronounce it. If it's a foreign name, every letter usually should be pronounced as written as otherwise it would not be there.
Hi, ___. What are you building these days? If what they are building is interesting, I ask more questions about it and decline to talk about what I do until I have a good understanding of what they do.
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Eventually the conversation will turn to you. Now you can say something like "I am building ___ that solves the ___ problem for ___ by _______________________."
This statement should be practiced and fluid. Ideally, you should tie to the other person's business. Try to keep it under 15 seconds. Then stop and do not continue unless the person you are talking to asks you more questions about it.
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I usually close the conversation with. "I think your ideal prospects are ___. Is that correct? ____ knows me and I'll send her an email about you right now, if that is OK. By the way, do you by chance know anyone who would benefit from using my product today?"
The key is to be able to add value to the other person immediately. You should be genuinely interested in learning more.
Simply categorizing your contacts and interconnecting them when appropriate also creates value. Just be careful to not create irrelevant introductions.
Unless the person you are talking to is representative of the target market, it is unlikely you will get validation. You may get informed of existing competition, which is valuable. I ask for referrals on the spot, but that takes practice to do smoothly.
I will go out of my way to not give you negative feedback. I've slipped a few times and it just made me feel awful. That is why if you wish to be talked out of doing something going to a place full of entrepreneurs is a wrong approach.
However you may want to think what you'll actually be asking people or what you want to get out of it. If you're trying to validate your idea - are we even your intended customer?
Alternatively you maybe looking more for some opinions on the technical challenges, to find someone to work with equally excited by the idea, info from people with industry experience, advice on the business side.
Ask the right kinds of questions and hopefully you'll get more than a general 'neat idea' kind of response.
Do you have any presentation material? Improvising is very hard to do. Its extremely difficult to convey a complex concept, and there's absolute no way they're going to understand it as well as you do. For this reason, there's no way they'll understand the value of your concept as much as you do.
Find the right venue. If you have an opportunity to give a formal presentation, then prepare for it. If not, you'll have to improvise. You can't demand other's attention, so you'll have to find the right time.