Honestly as an Author, fan mail is the best! I've only had the luxury of getting a few emails thus far, but it's always made my day. I tend to get more tweets because it's quick.
I think it's absolutely great to contact someone. Just use their email, contact form on their blog, facebook or something like that. Whatever is made easily available to the public. I wouldn't say track down their personal mailing address or phone number... that's probably taboo. But anything else is fine.
I've also got a few of my most admired authors on Facebook as friends, they are awesome and even comment on my status' once and a while, throwing out cheers or answering questions. It's the best.
My latest find is an author who I didn't know existed until I joined an online read-a-thon, next thing I know, we're FB friends, we've chatted back and forth here and there and she's even going to let me interview her for my blog when the read-a-thon is done! Very top notch of her!!!