Heroku, Rails 3.2.11, Assets.

In config/application.rb :

config.assets.precompile += %w( *.css *.js)
config.assets.initialize_on_precompile = false

In config/environments/production.rb :

config.serve_static_assets = true
config.static_cache_control = "public, max-age=3153600"
config.assets.compress = true
config.assets.compile = false
config.assets.digest = true

(list your Heroku apps) :

heroku apps

Enable the plugin (doc here), in your shell :

heroku labs:enable user-env-compile -a my_heroku_app

commit, push :

git add .
git commit -m 'assets install'
git push heroku master

Thanks @blakink