Just as I do in my business life, I always carry genealogy business cards with me. In addition to my contact information and web site address, my card includes the main surnames I am researching, the Nebraska counties I am researching and the European countries where my ancestors lived. It sounds like a lot of information, but believe it or not - it all fits on the 2" x 3 1/2" card. It's nice to be able to hand these to people you want to stay in touch with.
Since not everyone has their own genealogy business cards, you can carry some blank cards with you. I use the same cards that I print my own cards on - Avery Ink Jet 8871. Grab a few of the blank pages, snap them apart and carry them with you. When you want to exchange email addresses or phone numbers and your new found friend doesn't have a card of their own, they can write out the information for you on the blank card you've provided. When you get home, you can file the card with the rest of your business card contacts.
This technique also works with Rolodex card inserts.
Of course - this is for those of us who still use paper and pens and who are not part of the Blackberry generation who can enter the information directly into their smart phone!