Pavel Tsatsouline's The Naked Warrior focuses extensively on bodyweight exercises, and the one-armed pushup is his choice for a daily upper-body exercise. A couple hints from TNW:

• Use an incline. Do the pushups against a couch or a bench and gradually reduce the incline to keep the effort at a challenging, but not impossible, level.
• Experiment with the width of your stance. Wider feet will make the pushup easier.

Also keep in mind that this is going to be as much about full-body tension as it is about upper body strength. Even when doing easier versions of the exercise, focus on keeping your entire core tight.

Pavel Tsatsouline's The Naked Warrior focuses extensively on bodyweight exercises, and the one-armed pushup is his choice for a daily upper-body exercise. A couple hints from TNW:

• Use an incline. Do the pushups against a couch or a bench and gradually reduce the incline to keep the effort at a challenging, but not impossible, level.
• Experiment with the width of your stance. Wider feet will make the pushup easier.

Also keep in mind that this is going to be as much about full-body tension as it is about upper body strength. Even when doing easier versions of the exercise, focus on keeping your entire core tight.

Tips for maintaining tension:

• Tension starts with the core; keep the core tight by imagining a straight line between butt and belly button and tensing to shorten that line as much as possible.
• Your feet play an active part in that they should "anchor" or "root" to the ground; this doesn't mean to cheat by pushing but to feel rooted for tension.
• Breathing out naturally reduces tension (ask a competitive arm wrestler!) so don't exhale during a pushup.