The secret? Just grab a sheet of newspaper.
Take an old newspaper (or any kind of paper or fabric) and cut it to the size of your drawer. Bring the cutout along when you shop for inserts. With the cutout as your guide, you can puzzle together a really close fit with the inserts and cubbies right in the store — no returns required.
Of course, there might still be some gaps left over, even after you try the newspaper method. In that case, use those gaps for odd-shaped items, like rulers. Or add a few adjustable inserts, like these from Axis.