This isn't a very helpful answer for students at that level. It's necessary to say these words at some point, but without lots of examples students will have no idea what "good argument", "follows trivially from the last", or "counts as trivial" mean.

The only method I've seen work for a large set of students is to show lots of examples of what "good" and "not good" versions of "show your work" look like. Preferably on problems that look similar to the homework problems. And emphasize this during lectures.

E.g. after doing a derivation on the board: "by the way, this is what I expect to see when I say 'show your work' on a <insert problem type> problem."