Sane Music Management
Streaming service for most things
I loved Rdio, but since it's demise I moved to Spotify. I do most of my music listening there. Don't be cheap, pay for a subscription and avoid ads.
For music unavailable in streaming services, oldschool mp3 files
I buy music from Bandcamp. I keep these files stashed in an external drive, with CrashPlan backing it up daily.
A subset of these files are available as an iTunes library
I keep a small subset of albums on my computer. By doing that, I can play music if I'm offline and without my phone.
Keep offline files in your mobile phone for to-go airplane mode listening
I keep a good amount of files available for offline listening in Spotify. Make sure you also have a good noise generator and some headphones for loud situations. No iTunes integration at all.
Use Last.fm for stats
I have been using Last.fm for almost ten years, and it's always fun to go back there and see how my musical taste changed over the years. I have Spotify linked to it and the Last.fm application installed locally.