app.rb

require 'rubygems'
require 'bundler'

Bundler.require

unless production?
  require 'sinatra/reloader'
  # ..other non-production requires..
  
  # Avoid this one since it'll cause recursive
  # reloads when using other `also_reload` calls
  # also_reload 'app.rb'

  # Fine (unless those files are also_reload-free)
  also_reload 'app/**/*.rb'
end

# ... 

config.ru

unless ENV['RACK_ENV'] == 'production'
  require 'rack-livereload'
  use Rack::LiveReload
end

# your main app file
require_relative 'app'
run Sinatra::Application

Gemfile

group :development, :test do
  gem "guard-livereload", require: false
  gem "rack-livereload"
end

Guardfile

# Run `guard init livereload` to generate this file

# This is the minimal recommended setup
guard 'livereload' do
  watch(%r{^app\.rb})
  watch(%r{views/.+\.(erb|haml|slim)$})
  watch(%r{public/.+\.(css|js|html)})
end

Guardfile-asset-pipeline

# Don't forget to rename this file to `Guardfile`

# This Guardfile works with sinatra-asset-pipeline
# taking source and compiled extensions into account

guard 'livereload' do
  # source-compiled extensions
  extensions = {
    css: %i(scss sass),
    js: nil,
    html: nil,
    png: nil,
    gif: nil,
    jpg: nil,
    jpeg: nil,
  }.freeze

  # Files from asset pipeline
  extensions.each do |compiled_ext, source_exts|
    exts = Array(source_exts) + [compiled_ext]
    exts.each do |source_ext|
      watch(%r{
            (?:assets/\w+/(?<path>[^.]+) # path+base part
            (?<ext>\.#{source_ext})) # source extension part
            (?:\.\w+|$) # compiled extension part or nothing
                        # so it matches both .sass & .sass.css files
            }x) do |m|
        # We need to remap to send to the browser (livereloading)
        # the compiled version of the asset instead of the modified
        # one, that is, the source file.
        path = m[1]
        "/assets/#{path}.#{compiled_ext}"
      end
    end
  end

  # Will trigger full page reload
  compiled_exts = extensions.keys
  watch(%r{public/.+\.(#{compiled_exts * '|'})}) # Static files in public/
  watch(%r{views\/.+\.(erb|haml|slim)$}) # View templates
  watch(%r{app\/(helpers|models)\/(.+)\.rb$}) # app subfolders
  watch(%r{^app\.rb})
end