I just have a playlist of the couple hundred songs I'll generally want to listen to downloaded to my phone. If I want to stream another song or two it's not going to take that much data.
It's extremely easy to make a playlist available offline. Just go tap the little right triangle at the end of the song or playlist you want, and select "Keep on device." By default it is set to only download to the device over wifi, so be sure you are on a network when you do it.
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