The sole purpose of the Improve phase is to demonstrate, with fact and data, that your solutions solve the problem. Your organization will make changes in a process that will eliminate the defects, waste, and unnecessary costs that are linked to the customer need identified during the Define phase. Tools and strategies you will find for the Improve phase will include solution matrices that link brainstormed solution alternatives to customer needs and the project purpose, and methods for implementing desired solutions.
The tools most commonly used in the Improve phase are:

Brainstorming (covered in the Analyze phase)
Flow Charting (covered in the Analyze phase)
FMEA (covered in the Measure phase)
Stakeholder Analysis (covered in the Define phase)
Setup Reduction Queuing Methods for Reducing Congestion and Delays
5S's Method
Kaizen