Using the new Preset API to define a preset in another location from where you are actually calling `convert`.

sample_encoding.rb

require "rubygems"
require "bundler/setup"
require "ffmpeg" #=> https://github.com/brettbuddin/ffmpeg

include FFMpeg

Presets.define(:mp4_240x180_400k_aac, "mp4") do
  resolution "240x180"
  audio_codec "libfaac"
  video_bitrate "400k"
  video_codec "mpeg4"
end

convert("input_file.avi", :preset => :mp4_240x180_400k_aac)