Although this article specifically talks about Pandora, other radio services operate the same way.

Blatantly copied from http://ridetheclown.com

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From time to time, I hear people complaining about Pandora only playing the same tracks, or every station sounding the same pretty quickly. I ran into the same thing, a few years ago. However, I’ve gained some insights over time as to how Pandora works, so I figured I would write up a blog post on how exactly to avoid this situation. Using these rules, I’ve been able to maintain diversity in my Pandora stations without hearing the same tracks over and over. Item the first: The ‘Like’ button is more complicated than you think. ‘Liking’ a song does mean you like the track, yes. But it has an additional effect: You are telling Pandora you want to hear more tracks like this, on this station. What this boils down to: If you create a station for classical, and it plays metal, it doesn’t matter if it’s playing a song you love. You must downvote it. Or, at very least, not upvote it. Item the second: The ‘Dislike’ button is not evil, and it will not, in fact, consume your soul. 'Disliking' a track does not mean you never want to hear the track again ever on any station, it simply means Pandora should not play it or songs with similar attributes on this station. You must use it violently and often, or your stations will stagnate. Never using it makes Pandora lock itself into a loop. An example: I’ve been nurturing this wubwub station for months. Pandora occasionally plays rap instrumentals (no vocals) or hybrid tracks. Even though I usually like the songs, if I don’t smack Pandora with a rolled-up newspaper, it’s going to play others like them, which is bad, as they have the rap attribute. If I rate it positively, then their attributes will be added, which can drastically change a station. Finally: Use the 'Add Seed' and 'Add Variety' on their site only when the time is right. It can be your best friend, when done right. When you create a station, the song/artist/etc you choose to create from is set as the initial seed. When you add a seed, you are saying this track should be a foundation for the station. Ideally, seeds should be fairly similar – that is, don’t add a rock seed to a classical station. Adding seeds gives Pandora a bigger variety of tracks to choose from. A note about QuickMix: QuickMix is not shuffle. It does not play random tracks, it attempts to make a hybrid of the stations chosen. The above rule comes into play again: only select reasonably compatible stations, ie, don’t choose both a metal station and country. I’m not saying choose identical stations, but think, first, if they are compatible. Like, rock+dubstep = Pendulum, or electronica + pop = dance, and so on.

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