Beat me to it ... torque is a byproduct of displacement due to being able to inhale/exhale larger amounts of air (oxygen). Cheapest torque adder would be nitrous because it already contains more oxygen. Same goes for turbo/supercharger ... allows you to cram in more air. So, when it comes to making more torque, you have a few options:
1) Chips - optimize A/F ratio to make better torque, not much, but still more
2) Long Tube Headers - not really sure why they work so well at making torque, but my guess is they allow better cylinder scavaging, which allows for better cylinder filling.
3) Displacement - allows more air and thus more oxygen
4) Supercharger/Turbo(s) - crams in more air and thus more oxygen.
5) Nitrous Oxide - contains more oxygen than air thus allowing for an oxygen rich environment and thus more power.
In case you haven't seen a pattern, it's all about getting more oxygen into the cylinder. Do that and you will get more torque.
