I’m thinking of expressing my love for a dear friend through the magic of a mix CD. I’ve actually never done this before, sad to say, and I know there’s people who’ve pretty much made a science out of it. How should I go about doing it? What sort of stuff should I put on there?
The Who. Lots and Lots of Who.
And Kinks.
Follow Air supply with Scorpions. Always. It’s a gourmet combination like red wine & chocolate.
… Or …
You can mirror the progression of your feelings before & during courtship with songs that reflect your states of mind at those various times.
The Damned, Sex Pistols and Iggy Pop are usually prettry rich sources for me. But I’m old.
Love songs are good but don’t forget to include songs that mark important moments in your relationship, even if they are not necessaraly romantic.
The first song you danced to, theme song from the first movie you saw together (or one you both like), the song that played over and over again during your last road trip, songs from a concert you went to together, the song that was playing the first time you saw each other, or kissed, or realized you loved her…
Bonus points for a good, sappy explaination of why you included each song you did 
Once you think you have all the songs picked out and a general order, go through and listen to the whole thing, or at least the last part of each song and how it flows into the following song. Change up the order if you have to, just make sure that the pacing and transition from one song to the next is relatively smooth.
I hate nothing more in a mix than jarring changes in mood and/or style from one song to the next.
As far as content goes, it depends what kind of music you/they like, but try not to be too uniform. Continuing themes and similarities in genre are ok, but a good mix CD doesn’t sound like something you would order from an infomercial for Time Life’s Greatest Love Hits Collection Volume 17. I’m sure your relationgship has included a variety of feelings and experiences, and the songs should reflect that, as well as be put together with a recognizable progression from beginning, middle, to end if possible.