Regardless of whether you are an English teacher (as I was) or you teach math, science, social studies, or anything else, decorating your room is one way you have of connecting to students throughout the year. Even if you are a "floater" ask for a corner of the room to call your own, or decorate your rolling cart. Seasonal decorations are great - I always decorated for Halloween, Thanksgiving, Winter Holidays etc. If specific decor is discouraged, you can always decorate to suit your subject or a generic topic. Themes can connect to your subject and the season - for October : "The Mystery of _________." Fill in the blank with something from your subject. Edgar Allan Poe or Macbeth are good during this month for English teachers. World History might be studying the Middle Ages or Inquisition.
Seasonal decorations are inexpensive and available in many large variety stores like Wal-Mart or K-mart but I've found the best to be at local paper goods stores.
If seasonal decorations are not your thing, use student work to decorate the room. "Publishing" student work is a good way to help students feel that the room is as much theirs as it is yours and helps create a positive atmosphere. If possible, assign something that lends itself to products that can be displayed: posters that illustrate types of triangles; posters on types of musical instruments from the Renaissance; brochures on the importance of the rainforest; holiday cards from one character in a play to another such as a Christmas card from Lady Macbeth to her husband.
Nothing is more depressing that a room that is antiseptic and undecorated. By decorating your room or your space shows that you care about the atmosphere you create for the students.