Decrease exhaust resistance - Likewise resistance to exhaust will rob engine of power. Improved by adding second exhaust valve to the cylinder.To elminate back-pressure use big exhaust headers, tail pipes and free flowing mufflers. Giving that characteristic exhaust sound as it is less muffled.
Decrease weight - The less weight of every object in the car the less the car has to drag along.
Fuel injection - This injects a precise fuel amount into the cylinders thus improving performance and fuel economy.
"Chip" the car - Using super ROMS and chips the timing of the 4-stroke cycle is decreased and the engine produces a little more BHP.
Better camshafts - More efficient and better timed camshafts will also provide a little more BHP
Nitrous Boost - When you inject NO into an engine it does the following. Splits and creates lots of oxygen for more fuel to combust with. Thus more BHP. Also it has another advantage, when it vaporises it cools. This denses the gases and also it creates more oxygen due to the cooled air intake. However it is bulky and it is very expensive. Therefore is only used for a couple of minutes at max.