I've wanted to do this for a long time, but never had the gumption. Finally decided to tackle it.
Partial impetus from Lifehacker series that ran over two years ago:
Alpha Geek: Whip your MP3 library into shape, Part I - Level the volume
I went with mp3Gain like he recommended. It took the better part of a day for my 2700+ songs. I think in the future I'll just use the built-in leveling in MediaMonkey and make it a trifecta.
Alpha Geek: Whip your MP3 library into shape, Part II - Album art
Works pretty slick for importing album art. A lot of work, though (has to be worth it to you).
Alpha Geek: Whip your MP3 library into shape, Part III: Metadata
Not real satisfied with this method, mostly auto-tagging from Amazon. In some cases, the Amazon data was worse/less-preferable than mine. As I progressed, I used their data less and less and tweaked by hand more and more.
Further inspiration came from the recent Lifehacker post Alpha Geek: Whip your MP3 library into shape, Part III: Metadata.
It linked to a really useful post by Tippopotamus: MediaMonkey scripts to make your mp3 tags consistent. It might have been this post that convinced me to dive into MediaMonkey.
Boy, is it powerful. You sure can do a lot.
And Tippopotamus has many other good posts about music management. I totally plan on using his Dealing with new music regimen. Further, since I don't use iTunes (though he might convince me), I use an additional step he mentions: making sure I save mp3 files as [track] - [title]. Number one reason: My Creative Zen: Vision M doesn't use track tags, so, unless I save files this way, I can't listen to albums sequentially. And, when you've grown used to listening to certain cds (or even tapes!) in order, sometimes you'd like to again. MediaMonkey has a very handy Auto-Organize-Files function where you can easily rename files. I did this for all of the albums I might want to listen to in order.
Other interesting Tippopotamus posts:
Automatic mp3 genre tagging: is it possible?
Everyday tagging with MediaMonkey
Rating your music in iTunes easily, but thoroughly
Adopting iTunes for the uninitiated
If you've gotten this far: all his posts on mp3s
I still have a lot of trouble with genre. In general, I think simpler is better. Here's a post Tippopotamus linked on genre and subgenre: Organizing iTunes: Simplify your genre list.
It really is interesting what happens when my borderline OCD overcomes my very real laziness.
Result: 2747 songs, 11.7 GB, 189 hours, 7.87 days.
Still tweaking how they sync with the Zen, but everything's moving along nicely.