Better unit testing. Mocks. High code coverage. Fast test suite execution.
What are some of the things we need to get to these goals?
Some developers have an open mind to learn TDD. We need an effective mechanism to teach them the skills of how to think test-first in a fairly rapid manner. TDD is not just about test-first, it's about code quality. To ensure code quality, we need good tests. If your developers aren't writing good code, chances are they are writing even lower quality tests, if any at all. How do we get good quality tests that make sure our code is in good shape?
Test authoring, like most things in life, requires skill, and above all - Practice.
Take some well-written unit tests, and go over them with your developers. Teach them about the parts of each test, the setup, the test, and the assertions (and cleanup). Show examples. Have the experienced developers review the unit tests the other developers write. Have your test lead review unit tests too.