How can I make my demo louder?

When you mix down your track, you should have the master around -6 db as a peak. this gives you enough headroom for mastering it at a louder volume.
  
      I always run everything into a buss with nothing on it, and send that bus to the master for mixdown. What this does for me, is allow me to see there is no clipping going on before it goes to the master fader.

 Then on the master fader, I put a touch of compression on it. not very much, just enough to glue the tracks together.

  As for mastering, there are tons of ideas on how to do that. I am not saying my way is the best, but it works for me.
   I take stereo track and in a new project, clone it. ON the cloned track I put a compressor, playing with the controls so it is compressed somewhat middle of the road, but not to heavy.
 I then route those to a buss, with nothing on it, and that buss to the master fader.

   I normally use ozone3 for mastering on the master fader. The presets are nice, but they are just a starting point. there is no preset that works on each and every song to give it that polished sound. You have play with the different functions.

   I also use a dynamic range meter to get what my ranges are on the particular output. This helps a lot.

With ozone 3 or any of the vst's for mastering that comes with sonar, you can get to that "pro" sound, but you have to work with the presets and find it.