I recently contributed an article for carrying out Direct to S3 File Uploads in Python, as I have previously used a very similar approach to interface with Amazon's Simple Storage Service in one of my apps running on Heroku.
The guide's companion git repository hopes to demonstrate a simple use-case for this system. As with all of the Heroku Dev Center articles, if you have any feedback (e.g. what could be improved, what helped you, etc.), then please do provide it!