Think about a defense when you have completed a first draft of your thesis. Your timing will depend on committee member schedules and job market realities.
Allow adequate time for the revision process. When all committee members have read the first complete draft of your thesis, they will normally ask for revisions. Allow at least a month after you have circulated the revised version for committee members to read it. Normally, your supervisor will inform you if the thesis is ready to go forward to a defense, but it's worth asking all other committee members if they need more time or additional revisions. A reluctant committee member may signal a potential negative vote.
Schedule a defense, if you can, for a North American PhD in the middle of winter term of a year in which you are on the job market, ideally while you still have one to two years of funding left. With that timing, you can go forward to defense if you get a job offer, so that you can start out at Assistant Professor rank, but you can cancel the defense to hang on to your graduate funding if you don't get a job offer.
Double-check to make sure everything is in order yourself, even though most of the practical logistics of the defense are handled by your supervisor and the program secretary.
Make sure that all members of the committee have clean copies of the most recent revision of your thesis at least two weeks before the defense.
Verify that the room for the defense has been reserved and that everyone knows the correct date, time and place. Generally, the graduate program secretary will send out an email to everyone involved; if you haven't received one, consult with the secretary or your supervisor.
Prepare hard copy handouts and backups if you plan to use slides or other technology. Remember that technology can fail.
Plan to have a few hours free after the defense. Committees often take successful candidates out for celebratory meals or drinks.
Show up to your defense well in advance, with a copy of your thesis and notes for your introductory summary. As you may be asked to leave the room once or twice during the defense process, having some light reading, or a good cell phone game, to occupy you may make waiting less stressful.
Relax and review your introductory notes, as you may be ask to step outside the room at the beginning of the defense while committee members discuss procedure.
Summarize your thesis. Your short (five-minute) presentation should focus on the central conclusion you have reached and where it fits within the existing research in your discipline. Use clear, direct language, attempting to communicate the potential impact of your work rather than getting bogged down in technical minutiae.
Focus on responding to the questions, rather than going off on tangents, when each committee member asks you a few questions. This is not a quiz on specific facts, but an opportunity for you to explain the significance of your work.
Leave the room at the end of the defense if requested to do so as the committee deliberates. They will choose to accept the thesis as it stands, or more commonly with minor or major revisions, or reject the thesis. Once the thesis has been accepted, as most are, committee members sign the official paper work and offer congratulations. Your supervisor, at the end of the defense, is normally the first person to have the honor of addressing you as "Dr. X."