Lots and lots of decorative pillows piled on the bed will give in a warm and inviting feel. I like to prop four sleeping pillows upright to serve as the backdrop for all the decorative pillows.
A down comforter is one of the most inviting things you can add to a bedroom. I like to fold a down comforter at the end of the bed so it is plump in the middle. It adds a warm and cozy look.
A warm color on the wall is important. Paint is easy and fairly inexpensive.