Great post, SodPen.

NEVER EVER EVER (infinity evers on this one) download new media and just toss it directly into your default media library. Always have a second folder set up elsewhere that you can use as a temporary "quarantine" location for your media until you can run the tagging app on it and get it perfect, and THEN stick it in your library.
Great advice - I use an external drive (backed up REGULARLY!) to keep all of my tagged music on. My laptops internal drive has a folder called "New Tunes" where all of my downloads go. In the "New Tunes" folder, there is a folder called "Done". Once files in the "New tunes" folder have been renamed, tagged and mp3Gain'd, they go into the "Done" folder.

Only files in the "Done" folder make it onto the external drive.

I think I spent a day or two getting everything in place when I first seriously started my collection. Over the past few years, that day or two has easily saved me cumulative time that can be measured in weeks, most likely.
Church. Develop a system, and stick to it. There are LOADS of articles on organisation - you just have to develop a system that works for you and stick to it!