I guess that first off, I'd caution you to question whether you truly need a seed round. Investors tend to respond best when you're referred to them by someone they trust. If this is a major hurdle, you might find yourself working with some highly inexperienced investors or ones with bad reputations. From experience, I can promise you that it's a hell of a lot easier to bootstrap than to deal with a problem investor.
That said, try asking the question in a different way. How would you find customers if you didn't know anyone? Ultimately, that's your goal - you're trying to sell equity in your company.
I've always had good luck meeting important people. I say 'good luck' because I'm not sure that my method is actually any good. This literally might just be luck! However, some tactics I've used (with success) include:
1.) Contacting everyone I have worked with/for in the past five years, arrange a coffee/beer and ask them for help. Former bosses have always been particularly helpful (for me).
2.) AVOID meetups like the plague. In my experience, meetups are the best way to fill my calendar with utterly useless meetings.
3.) Talk to former professors. This worked wonders for me, but I have a business degree. Business professors are interesting people. Generally not terribly successful in business, they're armed with massive credentials and remarkably strong networks. I'm not sure how well this advice works in other fields.