General Motors didn't originally engineer the Duramax as a performance engine, but this V-8 diesel's bullet-proof reliability and power potential have since made it a darling among hot-rodders and long-haulers alike. A big part of the Duramax's appeal stems from the fact that increased power levels are so easy to attain and that they almost always go hand-in-hand with increases in fuel economy.
Apply all the basic hot-rodding tricks to increase airflow into and out of the engine. On the intake side, that means a high-flow intake tube and filter, a higher-flowing intake manifold and at least a gasket-matching job on the heads. A good down-tube and aftermarket exhaust system should take care of the job for most builders, but a set of headers will spool your turbo up a bit quicker and allow you to run numerically lower rear-end gears.
Install a quality power programmer or chip, depending upon the model year. Nothing you do to the engine is going to give it the instant power and versatility of a push-button power programmer. A programmer with external controls will allow you to dial up as much power as necessary when you need it and to set the truck for maximum fuel economy while cruising. If you're planning to hyper-mile the Duramax, consider having a custom program written for your truck.
Dump the 4WD if the vehicle has it and install skinny, lower-profile tires. The 4WD system is extremely heavy, and those big stock tires offer a great deal of rolling resistance. Dropping as much weight as you can afford to and reducing rolling resistance will allow you to run numerically lower gears without lugging the engine, which is good for this engine and its high-for-a-diesel 3,200-rpm torque peak. This course of action is not recommended for the Hummer H1, which -- when equipped with 2WD and skinny tires -- will almost certainly get laughed at.
Lower the truck and install a full-length belly pan and an aero tonneau cover. Consider attaching a piece of Plexiglas to the back of the top half of the truck's enormous grille. Make the cover removable so that you can get full cooling when towing. Lowering the truck will reduce frontal area and the amount of air that goes under it, the sheet-metal belly pan will help eliminate what air does make it under the truck; the aero tonneau (which tapers down from the cab roof to the tailgate) will drastically reduce drag and the Plexiglass will control airflow over and through the truck.
