[Jeordiewhite]

These things work great and are found at places like home depot or walmart.
http://www.webproducts.com/vent_register/
They can be cut into the right sizes and allow enough air flow or there are other types.
However there are some that help too and reduce less air flow but cost more:
http://www.xoxide.com/lascutfangri.html
I prefere this type tho collects dust better and easier to remove the filter only and clean it:
http://www.sidewindercomputers.com/fanfigu.html
DIY(this is the kinda idea with the heat register filters):
http://www.overclockers.com/tips103/