Here is some advice copy and pasted from other posts I've made on this subject:
I've taken a few groups to Oktoberfest in recent years and we usually book our rooms a year in advance. Lately we've been using the Motel One chain. Sure it's a chain and not very "Rick Steves"-y but it has what you need for Oktoberfest: a new, clean, safe, reasonably priced basic bed and bath. They have a great online booking system and you can cancel you booking with no penalty up to a few weeks before your arrival date.
I would also recommend staying along one of the main S or U-bahn lines that lead to the fest grounds instead of within walking distance. The main reason being price (its cheaper to not be within walking distance), the S or U-bahn is just as convenient especially if you don't have to change trains, and finally sometimes its better to not be near the fest grounds (less crowds, noise, spew, etc). We usually stay near Ostbahnhof (nicer neighborhood compared with near the Hauptbahnhof) and can take the U or S-Bahn lines to the fest (U5 goes right to the fest grounds).
To guarantee getting into a tent on opening day you will need a reservation. Each tent has their own reservation system and you have to purchase reservations for a whole table. But for opening weekend you are not likely to get a reservation as they are handled similar to season ticket holders where the previous reservation holders get priority. What we do is call a few travel agents who specialize in Oktoberfest and purchase their leftover reservations at a modest mark up. Also, check the toytowngermany.com Munich forums as people buy and sell table reservations there. Alternatively you can try to squeeze in opening weekend and with just two you might have some luck but no guarantee. The following Monday onward should be no problem.
An alternative to Oktoberfest is Munich Fruehlingsfest in the spring. It's like a mini Oktoberfest but without the high prices and crowds.
DJ