3. Forget the specifics
As you imagine the future, don't rely on any specific device or technology being a part of the plan. The hardware and software we have available to us today is hardly recognizable in comparison to that of five years ago. You can bet that trend will continue and even accelerate as time goes on. Building long-term plans around specific technologies that will likely bear little resemblance to their current incarnations only serves to put blinders on your creativity.
4. Define business requirements
Instead of focusing on individual technologies as a means to their own end, focus on the problems your organization will face and the requirements that will spring from those problems. As you do this, you'll find you've built a set of infrastructural requirements that can act as a yard stick to evaluate how emerging technologies can help you solve the challenges you face. Given a solid set of requirements, potential vendors will be far better able to suggest and provide solutions that meet them.
5. Implement toward the end state
As each budget cycle commences and you find yourself with near-term problems that need to be solved, evaluate each potential solution in the context of your long-term plan. If a solution doesn't have a place in your five-year plan, don't implement it. The problem at hand may be solved easily in a myriad of ways, but only a handful will address your organization's long-term goals.
In many cases, those solutions will carry a higher near-term cost or require more extensive changes to your infrastructure than the quick and dirty ones. However, weigh that increased difficulty and cost of deployment against the specter of needing to re-solve the same problem again next year or the year after. The best way to avoid reactivity in the future is to eliminate risks to your infrastructure before they have a chance to mature into real problems that need immediate resolution.
6. Refresh, reuse, recycle
As time goes on and you start to work toward the end state you've defined in your long-term plan, don't let it sit untouched. Every six months or so, revisit the future state defined in your plan and update it to reach out another six months into the future. The work of building and maintaining a solid IT infrastructure is never done; your planning can't be either.
Make sure to involve nontechnical business stakeholders in the process and get their input. They will often be considering business problems that will have a technical solution that you may be unaware of until they're requested. Likewise, involving stakeholders in your planning will cement IT's strategic role in their long-term plans for the business and put you in a position to become a trusted partner in developing them.
Breaking the cycle of constant action and reaction is extremely difficult. However, don't give up and resign yourself to a life full of fire fighting and last-minute decision making. If you build and continue to nurture a well-thought-out long-term plan, you will spend far more time building toward the future and far less time traveling in endlessly unproductive circles.