What we did is to explicitly list bringing any touched modules up to current coding standards as an explicit item on the definition of done. That way team members were expected to include any necessary refactoring and test coverage expansion in their story point estimates.
This is a great way to increase the compliance to coding standards and test coverage. If you can automate enforcement of this, it's even better. For example, your CI build could fail (or just send out notifications) if it detects that a modified file doesn't pass current coding standards.
Of course, it sounds like you have buy-in from management for these quality standards. For people who don't have this kind of buy-in for the whole team, they'll have to work individually to maintain quality.
