Because people who join dissertation committees essentially agree to do extra work in addition to teaching responsibilities, asking them as early as one year in advance can secure their help for your project. Send your prospective members an email asking if they'd be willing to serve, along with a brief description of your dissertation. For example, if your literature dissertation focuses on the role of gender in Shakespeare's plays, you might include what characters and works you'll focus on. Thank the faculty for their consideration and offer to supply more detailed information in the future.
Once you've heard back from your committee members, it's considered good academic etiquette to meet in person with them to discuss your ideas in greater detail. This meeting also gives you the chance to obtain their signatures for committee-approval paperwork. Don't attempt to barge into a professor's office and ask him to contribute or confront him during a casual run-in. Only meet with your committee members after informing them of your request in writing.