How can the Pentagon make sure it gets weapons as cheaply and as quickly
and as well suited for combat as possible?
Sorry for the politicized answers you've received. Some ways to achieve what you've asked is (in no particular order):
•to start with realistic goals—or realistic timeframes for goals requiring major advancements
•to notify companies who will be submitting weapons of all requirements ahead of time (rather than, say, suddenly canceling a program after years because they felt it needed to use Commercial Off-The-Shelf—when they never mentioned that in the specification before)
•to have stages of testing that allow brief breaks for companies to go and make improvements to their product and try again
•to have the testing accurately reflect planned usage with up-to-date input from active forces and not be heavily compromised by political goals (e.g., meeting NATO standardization agreements)
•To use modular technologies wherever possible