by which any system
One of the key requisites is knowledge of the current good and bad practises being undertaken within the process. Acquiring this knowledge is very difficult, very costly and absolutely key to improving the process.
A second path, is gaining knowledge about practises within organisations with comparable processes (not necessarily in the same line of business). Acquiring this knowledge is costly and only as reliable as the other organisations' ability to capture it. Many continuous improvement consultancies will assist you in gaining this information. It must also be assessed to see how applicable it is and whether it reflects accepted best practise.
The word continuous is also a key part of this method. One never gets to the end. The activities undertaken - assessment, comparison - must be repeated regularly.
Communication with the people involved with the process is very useful. They're the ones directly implementing the practises that you want to improve. Communication is two way. They can provide some very fine ideas for improvement as well as needing to understand changes being implemented.