It's good to clean up every once and a while.
Make sure you're on the master branch.
git checkout master
Make sure you're up-to-date.
Update remote list of branches and remove remotely deleted branches.
git fetch --prune
Delete all merged branches from your local except master. Adjust as needed if have specific branches needed for your workflow!
git branch --merged | egrep -v "(^\*|master)" | xargs git branch -d
Clear your git stash; if you set it aside months ago, you probably don't need it there.
git stash clear