When I do my radio show I plan nothing in advance.
I may pull a few CDs from my own library, but most of what I plays comes from the station and I make it up as a I go along. I start with a song and then go from there. Usually I try to avoid jarring segues, like from an upbeat salsa directly into some sad Celtic ballad. But other than that, there are no rules.
Because of this I'm always a bit surprised to go back and look at a show's playlist (tonight's show can be found here) and find unexpected trends. For instance, I seem to have gone heavily into Africa, the Middle East, and Latin music tonight. I have no idea why and it was certainly without malice aforethought. It just happened that way. The Alma Afrobeat Ensemble, Boubacar Traore, Rahim Alhaj, Rachid Taha...they all just sort of fell into my musical mind tonight.
That's one of the things I love about being a DJ at a college radio station, the freedom to play whatever I want. And finding the unexpected thread.
Except for specials (like my annual Thanksgiving week Native American show) there are no rules to my show. If I like it, I play it. And I have been known to stop a song mid-chorus when I discovered I didn't like it. Unprofessional? Sure. But hey, I'm a volunteer DJ and I refuse to play crappy music. And I do tend to go heavily into the upbeat. But when I get home and discover I've gone heavily into Celtic music or played a lot of Cuban tunes, it's always a nice surprise.
In the middle of my show I don't plan more than one song ahead, so I typically have 3-4 minutes to decide what to play next. I don't really put much thought into the theme, just "what would follow what is playing now?" And I do like to break things up so I don't do something like a Latin set followed by a South African set. It's more fun when you play Japanese pop and then lead into Finnish folk music.
OK, I'm rambling...but anyone searching for a dissertation topic might want to look into college radio and what drives a DJ to play, without intention, certain types of music on certain nights.