Though Munich has a vast number of hotels in all price ranges, many are fully booked year-round; this is a major trade and convention city as well as a prime tourist destination. If you're visiting during any of the major trade fairs such as the ispo (sports, fashion) in February, the IHM (crafts) in mid-March, or Oktoberfest at the end of September, make reservations at least six months in advance.
Some of the large, very expensive hotels that cater to expense-account business travelers have very attractive weekend discount rates -- sometimes as much as 50% below normal prices. Conversely, regular rates can go up during big trade fairs.
Munich's two tourist information offices -- at the main railway station and in the city center (Marienplatz, in the Rathaus) -- make hotel bookings. Telephone lines are often busy, so your best bet is to visit one of the offices in person.