Try practicing a couple of stories in a What, How, Wow format.
What: A short pithy sentence or two about what you did.
How: A list of 5-7 ways that you accomplished it
Wow: Three reasons why what you did was important. This should include both why it was good to the company (group,etc.) and what it meant to you.
An example:
I was the first student representative elected to the school committee.
I proposed to the superintendent of schools that we have a non-votintg, student member on the school committee. I gathered input from students, teachers, and school committee members. I wrote and presented the proposal to the school commitee. I led the group that came up with the process for holding the elections and the bylaws for the student advisory committee. I was one of 12 students that ran for the student representative office and was elected.
The school committee was pleased with the process and decided to make it permanent. When a grup of students walked out of class I was able to defuse tensions between students and the administration, make sure another walkout did not take place, and prevent students from being suspended. This also helped me overcome my fear of public speaking and encouraged me to pursue a degree in Eduacation.
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Having several of these ready can really help. Make sure that you focus on what YOU did. “I led a group that planned” vs “We came up with a plan that”.