What's the easiest way to insert a star nut into a steerer tube? I need to install one of those star-fangled nuts, but I don't do this often enough to warrant buying a special tool for it.  I usually manage to hammer the thing in eventually but in the past it has taken quite a while to get it in straight and once I even broke the nut.  

Is there a reliable DIY solution using cheap bits from the hardware store?  I am currently thinking perhaps try some PVC tube and a rubber bung.
The easiest solution is to use the right tool to make sure the star nut is aligned and set to the right depth in the steerer tube. I'd really recommend taking this to a shop and asking to borrow the proper tool. I own one, even though it only gets used once or twice per year. I've tended to go more towards compression plugs instead of the old star-fangled nut. If you're looking for an alternative to using the proper tool, you can thread a bolt in the nut and tap carefully with a hammer, or perhaps use a dowel or socket of the appropriate diameter instad of a bolt.