We bought our tickets in April or May of last year, shortly after ticket-only sales were opened up. At that time they were available via the official web site, but I believe that's no longer the case.

10 years ago, I was in Austria on business, and drove to Oberammergau looking for a "Zimmer frei" sign close to town, with the plan of getting up early in the morning and standing in line waiting for the ticket office to open up so I could possibly have the chance to buy a ticket someone else had bought and returned (the play was entirely sold out at that point). Driving through O'gau I saw a sign in front of a travel agency there advertising leftover tickets. I was able to buy a package that included a ticket and lunch, but no accommodations, at about 100% markup (but still much cheaper than the packages they're selling now, and a gigantic savings over buying a group tour package). Then I was able to find that "Zimmer frei" sign in nearby Ettal (or maybe I went to the Verkehrsbuero in O'gau or Ettal to find the room). That method is so irregular I wouldn't have recommended it that year, and things are handled quite differently this year anyway.

Also, even though thre are some packages still evidently available for sale (unusual for this late in the game - I think it's a combination of the economy and the big increase in prices this year), I did not see any empty seats at the performance I attended, so it may be that the available pickings are getting sparse.