If you are capturing the video over FireWire from a miniDV camcorder, the video data undergoes no change during capture or editing. (This is indicated by the fact that, as you edit, there will be neither a red or green line above your clips on the timeline until you add a transition or effect. Is this the case in your workflow? If not, you may be using the wrong project preset.)
YouTube, on the other hand, will compress and re-encode the crap out of your video -- and, unfortunately, that's beyond anyone's control but YouTube (which is why many people prefer to post their videos to sites like Vimeo or Muvipix). You don't say what type or settings of file you're posting to YouTube, but that can have an effect also.
Have you seen the FAQs on posting to YouTube to the right of this forum?
Along with Steve's suggestions and comments, if you DO have a red line above your Clips on the Timeline, you will want to Render for the best Preview (hit Enter). Also, to see the best display of your Video in the Program Monitor, change it from "Fit" to "100%." You will probably want to adjust the size of the Program Monitor to allow for this - better display vs having the image "fit" to the size of the Program Monitor.
Also, remember that with SD material, if you are viewing it on an HD monitor (like your computer), the image will not look as good as on an SD CRT TV. There is no way around this, as most computer monitors are much higher rez than SD material.