Joining in on the boilerplate fun, this is what I use when I don't want HAML.
$ git clone https://github.com/jondot/sinatra-html5boilerplate-erb-compass
$ cd sinatra-html5boilerplate-erb-compass
In one terminal:
$ cd public
$ compass watch #you need to have compass (`gem install compass`)
In another terminal
$ bundle install
$ rackup
Thats it.