I started being more cognizant of function overlap. No need to get a module, no matter how pretty it is, if all it does is perform the same function as a module I already own.
to a large extent this is true, especially of big shiny expensive modules... but functional overlap in utilities is handy... multiple vcas, mults, mixers, logic, clock dividers etc are extremely useful! especially once you've grown beyond a few voices... and are starting to look at multiple layers of control... not just controlling the shape, panning and sound of notes, but also controlling sections of 'songs' - bring in this voice for this section, add this effect for a bar every 4 bars...
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities