Can you suggest a way to keep changing the music on my iPhone?

My iTunes library is about cough 100Gb. The iPhone is a 16Gb model but let’s say that 8Gb of music is enough at any one time. At the moment I just transfer over stuff manually which means I end up listening to the same stuff which seems a bit of a waste as there’s probably loads of good stuff that I’d like but just never get exposed too.

So what I was thinking was to use smart play lists to pick stuff that I hadn’t listened to while still ensuring that I didn’t end up with 10 hours of rubbish.

Any ideas or sugestions on how I could do this?