What is the best methodology to follow to ensure continuous process optimization?

If you need a definite answer to your question you shoud read about process optimization methods and approaches since applications of optimization can be found in almost all area of engineering  ( process design, process control, model development, process identification etc ) 

To ensure continuous process optimization u can use several tools like : 

1- use of controls 

2- taking samples to lab for analysis 

3- scedule Tests 

4- use field operators