It's easy enough to get your IDP, and I think it would be wise for anyone planning on staying in Japan for an extended period of time to get. Even if you don't think you will get the opportunity to drive a private motor vehicle, in the case that you may have to at some stage it is worth paying the $20 (plus $10 for new passport-sized photos, if you've run out, like I had) to get it.
- Get a passport-sized photo
- Go into your nearest AA centre
- Fill out a form
- Pull out your driver licence
- Hand over the photo
You now have your IDP, ready to tackle the roads of... well, any country; but in our case: Japan.
Incidentally, I used my last ID photo for my Visa application, which is why I needed to get new ones ><