We'll often use the term "Future Proofing" when speaking about purchase and design decisions. What we are really talking about is Risk Management.
We want systems built and configured in such a way that you do not hit a wall that will force you to change systems, or go back to the original design decisions. Both of which are costly and disruptive.
Future Proofing is part art and part science, and is heavily dependent on expert judgment from your team. There are some things you know are coming, such as a planned application upgrade next year, and some things you won't see coming, such as a new reporting requirement or regulation. The former is easy to plan for, the latter... not so much.
To Future Proof for the unknowns, you simply want to be highly cognizant of anything that represents a constraint. Constraints represent the hard stop you want to avoid in the future. If you develop your ability to spot them, you can future proof without actually knowing precisely what the future will bring.
Constraints tend to fall into a few key areas:
If the solution needs to scale up, does the business case for the given solution scale up with it? Does it scale smoothly, or are there steps (e.g. outgrowing the family sedan with that 4th new baby)?
Obsolescence / Market Change
Buying a fleet of new VCRs for your organization in about 1999 would be a good example of this.
What are the limits on the database you're deploying? Can you add new types of data? Generally speaking, what are the costs associated with customization? How open is the vendor to integration with other applications?
An easy example of this is if you are building a new office and have the opportunity to install cabling. You can install inexpensive cabling, as it is sufficient for today's networks. You can also spend a premium to install higher end cable to allow for faster network equipment that we can predict will come in the medium term.
When governments change, could there be any impact? What if there is a new privacy requirement that dictates where data is stored? If the vendor has only one option for storing data, that represents a constraint, which is a risk that you are not future proofed.
With any risk management strategy, you will of course accept some risks. The important point in future proofing, is that you identify and analyze those risks.