How can I show the value of best practices?
I've faced resistance too. Here are some ideas:


Students would not turn in an assignment to an English teacher that was not properly punctuated and proofread, even if that teacher will be the only reader. They should not turn in an assignment to a CS teacher that is not properly formatted, documented, and tested.
Companies want to hire programmers capable of writing scalable programs. I can write a much stronger letter of recommendation for students who do well at this. (Adapt to college letters of recommendation for high school students.)
Rhetorically ask why people try their best when practicing CPR on a dummy.
When I interviewed job candidates for Google, I asked questions about testing. It would be hard for candidates to answer well without having practiced writing tests.
Give prizes for and share examples of student code that follows best practices. (Get a student's permission before sharing their code.)
Show them disastrous bugs that came from not following best practices, such as this one.