General Business: The Good...and the Bad

Alright, let Dr. Phil help you out :-) You seem to focus on the job search topic, but I'll try to keep it general.

First off, the follow up begins with the ending of the initial conversation. When you've met someone whom you have a clear purpose of following up with them in the near future, establish this when you're parting ways (eg. "I'll be sure to e-mail/phone you about X in a few days.". This a) makes you commit to doing something and b) establishes the expectation on the other end.

For the follow up, I would keep the message professional. Being serious or funny depends on your personality and the person you're interacting with. I wouldn't joke around with someone who didn't crack a smile the whole time you talked to them (despite your attempts to change that), but nonetheless seemed genuinely interested in a follow up.

As for e-mail or phone, that again depends on your comfort level and how you gauge the person you're conversing with. If they're someone who was glued to a Crackberry the whole time, e-mail is probably a good bet. But if they're the typical manager who's bogged down with thousands of e-mails, a phone call is likely a better choice, provided you're comfortable with on the fly conversations. Otherwise I recommend an e-mail (so you can script your response and change is as many times as needed), and follow up with a phone call if you don't hear back from them within a week.

If there's no clear immediate purpose, but you still think they're someone who is good to know, I would send an e-mail follow up a few days later saying it was good to meet them. In the e-mail, I would establish the potential value exchange that could occur (eg. "When your company grows, my company would be a great fit to help you with Y").

To keep the lines of communication open in the future, see if they use LinkedIn. If they do, exchange a connection with each other. I think it's a lot easier to remember someone that way.

Hope this helps!