Have you tried creating playlists and putting them on an SD card and using that? i assume you have the option of using the SD card below the screen?
So say 1 playlist full of your favourite songs
2nd playlist my be rap based music or whatever..
3rd Acoustic type stuff if your into that
4th Emo sh*t if your into that.. etc etc you get the drift..
Not sure as i don't have the RNS510 but thought i'd at least give you something to try as it doesn't take long to add playlists to an SD card then slotting that in.. Otherwise I'm sure other people who know more about the RNS can help your more