In the old days, a music collection was a pile of records or CDs (or tapes, or 8 tracks, depending on how far you want to go back!). I look back to those old days with some envy when I think about the the four different computers my household's music is spread across. Consolidating all the digital music into one shared drive is going to take a while - and that's not even taking into account all the CDs which should be ripped to really make it One Music Collection to Rule Them All.
So, the 43 Folders blog post on putting your iTunes library on a diet really appealed to me. It's pretty simple - weed out the stuff you don't listen to anymore, and delete. Especially if, like me, Destiny's Child tops your "25 most played" list. :) Or, if you can't bear to do that:
"... you may not have the guts to delete this stuff completely, but do yourself a favor and move the files to an external disk. Then put it away, mark down the date, and if a year later you haven’t touched it, delete that thing and use it for porn again like it was meant to be."