The most important modifications directly needed to support any turbocharged vehicle are related to fuel and ignition, even more so for a high boost one. The first and foremost concerns being intercooling, a/f ratio, and ignition. If the motor isn't properly tuned, it won't matter how strong your internals are, they won't last long. The charge air must be properly intercooled, this is not optional. Modifications to the fuel delivery relies greatly on the boost level and vehicle, and could vary from piggyback controller to standalone engine management, but in general rising rate fuel pressure regulators won't cut it for medium to high boost applications. Fuel injectors and fuel pumps will generally require consideration, unless your vehicle came with a high flowing fuel system. Ignition should be upgraded, and timing should be adjusted unless you have a knock sensor equipped vehicle, in which case the timing will be controlled by your ecu (unless you've replaced it with a standalone system).
Other important considerations are the cooling system, exhaust, and engine internals. Many stock cooling systems could be overloaded by a high boost application, that would be bad. Ideally the exhaust system would be as non restrictive as possible, technically you don't even need one after the turbocharger, just a straight pipe out to the back (legally may be a different story).
One would also be wise to take the engine internals under consideration when building a high boost motor, but again, it depends on what motor you started off with and how high the boost is...not all engines were created equal, and some may be just fine left alone. As a rule make sure the motor is in good shape, that is the absolute minimum requirement, and I have heard of some well tuned high boost motors lasting quite long with stock internals...but the key word is well tuned. Basically, make sure all of the internals can handle the added heat and stress. What you actually need to beef up is completely up to your discretion, and depends on how wild the boost is going to be.
Getting the HP to the ground reliably is a whole other story.
I'm sure I'm overlooking some stuff since this is such a huge subject, but I'm sure somebody will come and add in anything I may have missed.