Its no secret. I almost feel embarrassed writing this. But the truth is the majority of swimmers forget this one little detail.
What is it? Its easy:
If you want to swim faster in meets, swim faster in practice.
Yes, that's the number one best way to improve.
The easiest way to do this is to practice with somebody else your same ability level. Get competitive. Don't let them finish first. That old dude across the pool? Beat him into the wall. Making your intervals? Make them by more. Any chance you can, turn up the heat in practice. You won't be disappointed.