Train honesty in situations where lying would barely matter.

The low-stakes lie is where honesty is actually built or lost. There is no pressure justifying it, so the choice is clean, and whichever way you go teaches you what kind of person handles these situations. It also keeps the reflex in condition for when a lie would genuinely help you.

Catch yourself in the next small convenient inaccuracy, the vague excuse or the flattering version. Say the plain thing instead. Nothing will be at stake, which is the point.