1) Every three months whoever is in charge (me right now) starts a post for suggestions. Everyone is free to post and "second" as many suggestions as they like. I tally up all of the suggestions and seconds and chose ten books as possibilities. I base those ten on which suggestions are the most popular and which will likely generate good discussion. I also try to choose books from a variety of genres and styles. I don't include any suggestions that have been read before and I typically don't include suggestions that have less than two votes in the suggestions post. I also don't include suggestions from any threads, since I don't read every thread.
2) I start a second post where I list the most popular/best discussion books with links to their Goodreads pages. Everyone is able to list their top three choices from those ten books. I don't do a clicky poll because we don't want "drive by" votes. I also ask that only people who plan to actually read and discuss vote, so we don't wind up with a selection that people who don't plan to read chose. Then I tally the votes and the three most popular books are selected. I try to space them out so that the most recent releases are at the end of the time period and that the genres are varied (not two YA or non-fiction in a row, for example).