1.)Commercial space approach. SpaceX and now Orbital Sciences have shown that as much as 90% off of the development cost can be cut by the cost-sharing of the commercial space approach.
Developing a rocket (or anything else on Earth) does not get cheaper automatically, because a private company is doing it. The same forces that interfere in public spaceflight are also existing in private spaceflight.
SpaceX is actually a pretty bad example for a commercial approach (For example, they are also sharing NASAs "Not-Invented-Here" attitude), Orbital Sciences is the older and better example. SpaceX just has the better PR department.
if you want to put the R&D costs into relation, look at the Delta II development, which, while happening pretty early in history, had been a pretty cheap and cost-effective program, until the much more commercial oriented Ariane 4 hit the market.
2.)Go small. NASA’s SEV weighs about a third that of Orion. Orbital’s Cygnus weighs about a quarter. Imagine how small, and low cost, your lunar mission could be if you only had to transport a quarter of the mass to the Moon.
If you need a truck, you should use a truck. It makes no sense to drive alone to work in a SUV, but it also makes no sense to buy large furniture with a Fiat 500.
What you can do is develop in small steps to keep the costs low. That is already scientifically documented, how working with small agile steps reduces costs by reducing the inertia of the program. But that does not always work that easy in spaceflight as in other industries.
3.)Use existing components. The huge development costs for the Apollo program and of Constellation were because they had to use all newly developed components. Those costs would be reduced greatly if you only had to adapt already existing components. No Saturn V, Ares V, or SLS, and their huge development costs, required.
Wrong - old components can be cheap in procurement, but result in the much more expensive system. You have to see the big picture in such a program. It can pay out to develop the right components for your system new, and use commercial parts where you can do that for your advantage.
Usually, you have better chances to use commercial components for less specialized roles. A navigation system does not need to be developed again, if there are already dozens of suitable systems on the market and technical standards around that permit swapping such components even late in development.
4.)Use international partners. The cut in development cost by engaging in cost-sharing is already included in the commercial space approach. However, the cost to NASA can be cut even further by sharing development costs with our international space partners such as the ESA and Japan.
That does not make things cheaper. Rather contrary. Different cultures cause friction as well.
The advantage of international cooperation is bringing different strengths and capabilities together. The USA might have been able to build their own spacestation on day in the distant future, but cooperating with the Russians, who had much more experiences in that field made it much easier to get ahead. But the Russian approach had its own limitations, so both sides needed each other to complement. ESA, CSA or JAXA are rather small partners, but also contributed important components that made the ISS possible without Russia or USA having to reinvent the wheel.
But cheaper was never the result of this in total. Russia could have developed the computer system for their segment themselves, but having the Europeans was more effective from a technical point of view, but added costs for the cooperation.