One of those ways, of course, is my iPod, and if you've been trying to figure out how to get your iTunes listens scrobbled into Last.fm, worry no more: Scrobblepod is here to save the day. It's a go-between for iTunes and Last.fm, and allows you to sync your iPod listens up with Last.fm before your iTunes listens get entered, ensuring everything gets listed just fine. Now, people will be able to see that you've listened to Mamma Mia whether it's on your iPod or your iTunes install.
Of course, I still haven't seen a way to track the other way I listen to my music-- I keep all my actual music files on my Powerbook, and share them directly to my desktop through iTunes. That way, I have access to my music no matter where I take my laptop. Unfortunately, neither iTunes or Last.fm treats listens through shared libraries as if they were "real" plays, so no matter how many times I listen to that Timberlake Justice remix, it'll never show up on my "Recently Played" list, or in Last.fm.
But anyway, my problems are complicated. If all you want to do is track your iPod listens into Last.fm, Scrobblepod's got you covered.