If you don't already have a Heroku account, create one, it's free. Then install the Heroku gem.

Basic Octopress setup

First make sure you've installed the Heroku gem

gem install heroku

Next create a heroku app for deployment. If this is your first time using Heroku, this command will ask for your account credentials, and automatically upload your public SSH key. If you don't already have a public key follow Github's guide and create one.

heroku create

This will create a new Heroku app for you to deploy to and add a git remote named 'heroku'.

# Set heroku to be the default remote for push/fetch
git config branch.master.remote heroku

Edit the .gitignore in the root of your repository by removing public and Gemfile.lock. This will let you add generated content for deploying it to Heroku.

rake generate
git add .
git commit -m 'site updated'
git push heroku master

That's it, you just deployed to Heroku. If you want to set up a custom domain check out the docs at Heroku.