I've been participating in Booking Through Thursday for a few months now. I think all spring actually. Anyway, you just visit the site and find out the question of the week. You answer it in your own blog and then go and leave a link back on the original post. It's fun. I don't participate each and every week. But when it's a good question, I like answering and participating.
Sunday Salon I am still brand new. I just learned about how to "officially" join last week. I found out through the A Novel Challenge blog. There's a site where you fill out a form (name, blog address, etc.) and you're added to a blogroll list of participants. Your commitment is to post on Sunday about something bookish related. I don't know if there are specific themes or not. But anything related to books seems to be okay. It *seems* to be an informal gathering where book conversations happen.
These are book memes. It is simple to get involved. If you don't want to participate one week you don't have to. Basically you copy the badge picture for your own post. You copy the question into your post and answer it. Then you go to the blog hosting the meme and post a link to your answer in the comments. After that people visit around to see other peoples answers.