http://blog.darkhax.com/2010/07/30/auto-scale-your-resque-workers-on-heroku

Gemfile

gem 'resque', '>= 1.10.0'
gem 'heroku' # You will need the heroku gem for this too.

heroku_resque_auto_scale.rb

require 'heroku'

module HerokuResqueAutoScale
  module Scaler
    class << self
      @@heroku = Heroku::Client.new(ENV['HEROKU_USER'], ENV['HEROKU_PASS'])

      def workers
        @@heroku.info(ENV['HEROKU_APP'])[:workers].to_i
      end

      def workers=(qty)
        @@heroku.set_workers(ENV['HEROKU_APP'], qty)
      end

      def job_count
        Resque.info[:pending].to_i
      end
    end
  end

  def after_perform_scale_down(*args)
    # Nothing fancy, just shut everything down if we have no jobs
    Scaler.workers = 0 if Scaler.job_count.zero?
  end

  def after_enqueue_scale_up(*args)
    [
      {
        :workers => 1, # This many workers
        :job_count => 1 # For this many jobs or more, until the next level
      },
      {
        :workers => 2,
        :job_count => 15
      },
      {
        :workers => 3,
        :job_count => 25
      },
      {
        :workers => 4,
        :job_count => 40
      },
      {
        :workers => 5,
        :job_count => 60
      }
    ].reverse_each do |scale_info|
      # Run backwards so it gets set to the highest value first
      # Otherwise if there were 70 jobs, it would get set to 1, then 2, then 3, etc

      # If we have a job count greater than or equal to the job limit for this scale info
      if Scaler.job_count >= scale_info[:job_count]
        # Set the number of workers unless they are already set to a level we want. Don't scale down here!
        if Scaler.workers <= scale_info[:workers]
          Scaler.workers = scale_info[:workers]
        end
        break # We've set or ensured that the worker count is high enough
      end
    end
  end
end

scaling_job.rb

class ScalingJob
  extend HerokuResqueAutoScale

  def self.perform
    # Do something long running
  end
end