avatar_uploader.rb

# app/uploaders/avatar_uploader.rb

  process :fix_exif_rotation
  process :strip
  process :resize_to_fill => [1024, 768]
  process :quality => 90 # Percentage from 0 - 100

carrierwave_initializer.rb

# config/initializers/carrierwave_initializer.rb

module CarrierWave
  module RMagick

    # Rotates the image based on the EXIF Orientation
    def fix_exif_rotation
      manipulate! do |img|
        img.auto_orient!
        img = yield(img) if block_given?
        img
      end
    end

    # Strips out all embedded information from the image
    def strip
      manipulate! do |img|
        img.strip!
        img = yield(img) if block_given?
        img
      end
    end

    # Reduces the quality of the image to the percentage given
    def quality(percentage)
      manipulate! do |img|
        img.write(current_path){ self.quality = percentage }
        img = yield(img) if block_given?
        img
      end
    end

  end
end

FYI, how to do this in just an uploader class: http://stackoverflow.com/a/4767021