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.
Accessories in general add warmth to a room, but you can switch out your accessories to showcase only ones that reinforce the cozy atmosphere you are creating.
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.