app.rb

# This is our base controller, much like ApplicationController in Rails
class App < Sinatra::Base
  # Default settings that should be shared by all controllers
  set :foo, :bar

  def shared_helper
    # this will be available for everything
  end
end

class Users < App
  get '/' do
    ...
  end

  post '/' do
    ...
  end
end

class Comments < App
  get '/' do
    ...
  end

  post '/' do
    ...
  end
end

config.ru

require 'app'

map('/users') { run Users }
map('/comments') { run Comments }