I keep all my code for HtDP in a directory together, e.g.,
htdp/
htdp/code/02-01-01-explore.ss
htdp/code/02-01-02-tan.ss
htdp/code/02-02-01-temperature.ss
[...]
When I need something, I grep through the sources from within Emacs
and grab what I need. The *grep* buffer lets me see at a glance all
the files that reference something I've built before. I often use
Emacs side-by-side with DrScheme.