Most people like to decorate a room, and then they re-decorate it after a year or 2 to keep it from looking stale. If you're one of those, join the club.
There are a number of changes you can make to a room to give it a whole new look, without draining your bank account. Paint is one of the biggest changes to a room that costs very little. Use drastically different colors, or experiment with unique paint treatments (sponge, faux finishes, brush, rag, pearlescent hues, etc.)
Another easy and inexpensive change is the accessories and knick knacks in the room. You can find vases, pictures and table decorations at any home store, craft store, antique store or gift shop. Donate your old accessories and start with a new set.
Plants and flowers can also refresh a room for minimal cost. Whether its live or fake, find greenery that fits your style without being overbearing.
Some window treatments can also make a statement in a redecorated room for very little. If you have old blinds or shades, replace them with new models, new colors and patterns. There is a great variety of looks to fit any decorating style. Shutters can also dramatically change the look of a room and will last a long time.
Whatever elements you choose, enjoy the experience... because you're just going to change it in a couple years anyway!