The cheapest way is to make the car lighter. Take out the inner fenderwells, remove the back seat, carpet, and sound deadener. Replace the front seats with a pair of plastic or aluminum seats (make sure you mount and brace them correctly), move the battery to the trunk, remove the front sway bar, and get all the non-essential components out from under the hood.

Every hundred pounds you remove is good for 1/10th of a second lower ET!!!!