Happy New Year!
There are a lot of ways you can participate. First things first: go to the IYA2009 website, and spend some time poking around there. That is the central hub of the effort, with a huge amount of information on how you can participate.
My personal suggestion is to throw a star party. Invite your neighbors to see the planets or the Moon through a telescope (if you don't have one, find someone in your area who does). Joining or just contacting an astronomy club would help. I have links to find one near you on the old site.
A star party might take a while to set up, but there are things you can do right now. There are so many online activities! There are three I particularly like:
Cosmic Diary: "The Cosmic Dairy aims to put a human face on astronomy: professional scientists will blog in text and images about their lives, families, friends, hobbies and interests, as well as their work, their latest research findings and the challenges that face them." There are several posts up already.
365 Days of Astronomy Podcast: "a project that will publish one podcast per day, for all 365 days of 2009. The podcast episodes are written, recorded and produced by people around the world." And by people, they mean you. This is a participatory effort, where anyone who wants to can submit an entry. Head over there, pick a day, pick a topic, and record your cosmic thought! And the first one is already up. I love the intro music; it's by skeptic and science junkie George Hrab.
If you're a regular reader here, you know how much I love astronomy, and how much I love to share that love. So join me, and join all my friends, colleagues, and fellow sky-lovers. Share astronomy with someone!