My best friends are people who listen.
When I first start to become friends with people, an early sign that they're a keeper is that they demonstrably listen to me. And I'm not saying I want passive listeners who become a sponge while I yammer. I like when I find out that not only was the person listening; they were synthesizing information, using it to understand me, and applying it to our further interaction.
Recently on Facebook I came across a husband/wife quiz where each member of the spousal arrangement is supposed to answer for the other. It's mostly cute little questions like "Which of these two things would she like better?" or "What's his middle name?" or "If they've had a bad day, what would you expect them to eat afterwards?" Answering well requires active observation and receptivity when someone talks to you. I found myself thinking I would be able to answer these questions pretty successfully for my best friends, and for the most part they would be able to do the same for me.
They give good gifts. This is something I appreciate not because I want stuff, but because I know it means they were listening to things I said and incorporating those facts into an understanding of who I am. It's not just a trivia quiz--oh, she likes licorice, coffee, and the color yellow. It's about observation--oh, she's always cold so I'll get her a cool blanket. She puts cute things in her hair and wears funny socks so I'll get her things in her style. She reads this author so maybe she'd like this one.
I make myself a really open book most of the time and am eager to talk about what interests me. I love when it's clear they are thinking about what I've said and when they add new information. It's so cool when someone reads one of my books and can talk to me about who the characters are or ask complicated questions. It's amazing to be able to discuss entertainment or social issues in depth with someone who's paying attention to the same things I am and has the same values. And it's so cool when talking to the other person doesn't feel like a trade--like first you talk, then I talk, and we've both had our turn. I've met people on the social websites I go to who are like that--they're actually just waiting for their turn to talk, and the questions they ask are obligatory.
The people I love talking to are here because they want to be, and they are giving as much as they take.
Asking about their lives and interests, getting pulled into what they love, getting invested in who they are, is not a chore or a project for me. It's a natural process that happens much more easily when I know that person is listening.