Conserving Forces
The principle behind conserving your fighting forces is that each unit will continue to fight at 100 percent efficiency until it is killed. A unit with but a single hit point remaining will continue to fight like new. To this end, do everything you can to keep all your units alive as long as possible. Keep your healthier forces toward the front lines and leave your damaged troops in the rear so that they can continue to fight without fear of being killed. When overseeing groups of ranged units in combat, take the time to maneuver those units taking the brunt of the damage in front to the back of your force, so that damage is distributed between your forces rather than concentrated upon individuals so they die off. This way, you will continue to attack with maximum force for as long as possible.
All three races have a means of restoring their injured units back to health. The Terrans may quickly repair any mechanical unit (including spacecraft) by assigning one or more SCVs to the task. Multiple SCVs repairing a unit will get the job done that many times quicker. Repairs cost a small quantity of minerals and Vespene Gas if applicable. All Zerg units regenerate slowly back to full health if they take damage. Try burrowing injured Zerg ground units underground to let them heal up in between battles or fly your aerial units to the safety of your headquarters and let them regenerate there. While the Protoss cannot restore lost hit points, their shields operate much like Zerg hit points in that lost shield points gradually regenerate. The Protoss can greatly expedite this process by means of a Shield Battery structure, designed to restore lost shields almost instantaneously.
Do your best to keep your forces alive as long as possible, and do not neglect to restore them to health if you have the opportunity. You will save valuable time and resources in this fashion.
Resource Management
You must never find yourself waiting to save up for individual units. Do not rush to build a Terran Science Facility and Physics Lab so you can just barely produce a single Battlecruiser with your last ounce of resources. Instead, allocate your wealth to build great numbers of the best units you can most easily afford. Only when you have vast reserves of minerals and Vespene Gas should you begin producing expensive units like Battlecruisers. Purchase deliberately and resist the urge to rush up the technology tree when dealing with a human opponent. Remember that low-level units in large groups tend to be far more effective than few numbers of more expensive units.
In general, try to spend at a rate just slightly lower than what you earn. Continue to build profit so you can amass plenty of reserves should you ever need them (expect that you will), but do not restrict yourself by building fewer units than you can afford.
Next: The Application of Theory - Execution 
