The trick is to move very slowly and deliberately. Their eyes and brains are wired to detect and react immediately to any large, sudden movement, but they are not quite so spooked by very slow movement. (edit: backing away quickly is therefore as likely to spook them as moving forward or sideways quickly)
Telephoto lens on my cell phone doesn't hurt, either.
And he's kinda used to me being about in the garden, so that might make him a little less jumpy.