Why is one bedroom always colder than the rest of the house?
For a quick and dirty fix, I have had some success with a Register Booster Fan.


This has alleviated most of the problem in my house.  I still plan to install more insulation (and upgrade to a multi-zone system), but this is a lot less work up front.  You can use it as a stop gap until you figure out what permanent fixes you need to make.