The method to achieve focused improvement is the use of the five thinking processes.
Most of the popular performance improvement methods, such as TQM or reengineering are not focused improvement methods, i.e. they do not distinguish working on the factors that limit system's perfomance (constraints) -- which brings immediate results, from working on non-constraints -- which does not bring any results, only increases costs and distracts management's attention.
See also: Donella Meadows' twelve leverage points to intervene in a system