I once spent weeks, maybe months, putting together a set of mix tapes. This memory was triggered by a post about boomboxes on Flint Expats (http://www.flintexpats.com/). I had a great Panasonic boombox, btw.
I spent abundant time studying how each song ended, to make my segues invisible (e.g., if one song ended on a drumbeat, the next would start with a beat or note that followed logically, rhythmically, musically as the next sound). I mastered the input level knobs so that I could create a fadeout early; I studied fractions of a turn on the counter wheel to reduce dead air to nothing; I studied Trouser Press music guides to discover what could be posited as the absolute best music released in a given year.
I made these tapes for every year between, I think, 1977 and 1989. I gave a set to my husband-to-be when I met him in 1990. I mailed them out to everyone whom I considered a friend. The project took me enough time that, even now, I ask myself, "Would I include this song on a 2009 mix?" It was "a moment" (as in, validating all that time spent) when a friend (who tragically died in 2001) told me, ca. 1998, that he won a bet by knowing that a film-soundtrack song, which became a minor hit, was a re-release, because he'd loved it and played it often on one of my 80s mix tapes. (It was a Proclaimers song.)
What is your best boombox and/or mix tape memory?