I've been a Radio Producer slash Disk Jockey since 1979, and was making tapes for myself and others long before that. Ever since the minnehaha K Tapes in the mid-80s, I've kept track of every mix tape and CD made. These are different than Shockwave Distribution tapes/CDs, and are separate from various requests that have come down the pike. While there are no "rules" per se, here are the guidelines I usually follow for making mixes, more-or-less in this order:
Prep: The ideas usually start with a comment or (as in the case of kalimac) a moderately extensive list of their taste. Sometimes the subject is specific to a topic, eg the CD made for mle292 with Minnesota music. Sometimes it's more generic in a genre I'm not conversant in, and it takes a while for me to collect enough music I think it worth, eg the year or so I took to assemble cari_rose's Country mix tape. Sometimes I'm making suggestions for a musician. Sometimes I'm simply playing dj and spreading my own tastes, eg the CDs I've made for nieces and nephews. This is a tricky assessment, and sometimes I just guess.
Meta purpose: I make a mix that I would like to listen to, or at least can stand to hear. With some exceptions, my Hidden Agenda is to play dj and proselytize for my favorites.
1. I try to avoid music that the recipient already has, and usually try for songs that they've never heard. This is the hardest -- and least successful -- aspect of the selection process. I rarely have access to their full music library or have time to grill them on an individual song. I always have mixed feelings when someone says, "Oh, that song is one of my favorites! The artist has been one of my favorites for a long time!" On one hand, I'm pleased that I accurately pegged their taste in music. On the other, I'm disappointed that I didn't introduce them to a new song. I'd rather be an FM dj than an AM dj.
2. I'll include a specific song or group, if they request such... and I have it. I have a medium sized CD collection (about 1200) with some access to other digitized material, but that's a very small percentage of recorded music.
3. Failing that, I'll try to find songs that are similar to songs or in the style of groups that we've discussed. This is the most fun to do.
4. Failing that, I'll make leaps of imagination into related styles and associations. This is the most satisfying when it works and I've learned not to invest too much into guesses in case it doesn't work. Gary Farber once described my mix tape for him as "a mixed bag". More recently, lydy was surprised that she liked all but one song on the first CD I made for her. I'm much better now than before, but it's still a bit of a gamble.
5. Failing that, I'll fill in with my own favorite songs. I'm playing dj, establishing common ground and proselytizing. And if it gets played while I'm around, I'll enjoy it all the more.
When I was making tapes, I took pride that I filled the tape, and almost always had less than a minute remaining per side. When CDs were a max of 74 minutes, virtually all my CDs were at least 73 minutes. That was fun to do, but hard. When standard CDs went to 80 minutes, I gave up and am usually satisfied when it has at least 74 minutes. I hate to burn CDs of less than an hour.
On occasion, I'll hear a song that person X would like or that they really need to hear. Sometimes I have to go out and get the song. When mplscorwin turned 18, I figured he really, really needed to hear "Billy the Mountain" by Frank Zappa. Everybody should hear "Billy the Mountain" at least once in their lives. I constructed a whole mix for laurel around "Dance In The Smoke" by Argent. When my kid friends request more lukeski I take the opportunity to spread Dementia artists around.
And, lastly, whim. Whatever I feel in the mood to hear (or at least play with the file) that day. Every now and then I'll just guess. It'll usually be a song I like but don't love (yet), often new (to me) but something I feel will fit in the mix. This actually works out okay, since my tastes are idiosyncratic enough that stuff on my edge is on other people's edge too. Plus, after nearly 30 years of radio, I'm a pretty good guesser.
Then I put them in some sort of order. This is the hardest part. I'll try to connect the songs: thematically, musically or whatever. I can't always connect all the songs, or even have all of them segue from one to the other. I'll frequently put an instrumental at the end of a sequence to break up a thematically linked set, but sometimes I just go from one unrelated song to another. Starting well and ending well is important. Still, the order is secondary to the presentation of new songs. Sometimes, diversity is it's own reason.
Making CDs is significantly easier than making tapes. iTunes -- at least having hundreds of songs in aiff format on my computer -- makes CDs very easy to generate. I can probably do kalimac's CD with songs already on my system. Though, now that I think of it, one of my preferences is to force myself to add a song to iTunes. After a year and a half, the songs in my iTunes library are a good representation, and I'm doing that less and less. Of course, I add new songs as they come in. The ones I like, anyway. And I'm still nearly 200 CDs behind in listening... but that's another post.