You can use `git merge --squash` for this, which is slightly more elegant than `git rebase -i`.  Suppose you're on master and you want to squash the last 12 commits into one.  First check that `git status` is clean (since `git reset --hard` will throw away staged and unstaged changes) and then:

    # Reset the current branch to the commit just before the last 12:
    git reset --hard HEAD~12

    # HEAD@{1} is where the branch was just before the previous command.
    # This command makes the state of the index to be as it would just
    # after a merge from that commit:
    git merge --squash HEAD@{1}

    # Commit those squashed changes.  The commit message will be helpfully
    # prepopulated with the commit messages of all the squashed commits:
    git commit

The [documentation for `git merge`](http://www.kernel.org/pub/software/scm/git/docs/git-merge.html) describes the `--squash` option in more detail.