Write good unit test

https://developer.salesforce.com/page/How_to_Write_Good_Unit_Tests

http://www.c-sharpcorner.com/article/automated-code-coverage-and-unit-tests/

https://softwareengineering.stackexchange.com/questions/272794/how-do-you-check-that-code-has-been-covered-automatically

https://salesforce.stackexchange.com/questions/41755/is-the-75-code-coverage-requirement-to-deploy-to-production-per-class-or-overal