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.
WARNING: First make sure you commit your work—check that git status is clean (since git reset --hard will throw away staged and unstaged changes)
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 sets 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 describes the --squash option in more detail.
Update: the only real advantage of this method over the simpler git reset --soft HEAD~12 && git commit suggested by Chris Johnsen in his answer is that you get the commit message prepopulated with every commit message that you're squashing.
