Buffering Long TTL Lines
Oh boy, where to begin...

Basically, there is this whole field called "signal integrity" that directly relates to what you are asking about.  SI is a complex topic that takes years to really comprehend and wrap your head around.  There is a lot of information on the net about SI, and honestly most of it is wrong, misleading, or misguided.  Unfortunately, knowing this stuff is essential to getting digital systems to be robust and reliable.  Every EE should know this stuff.

The best guide for this is the book High Speed Digital Design: A Handbook of Black Magic.  A highly recommended book, and worth every penny!

There is no way that I can give you a good overview of the things you need to know about SI.  The topic is that big and complex.  But let me give you some random bullet items that hopefully will, with a good book, point you in the right direction.  


Voltage drop across the traces/cable/wires isn't normally an issue for a digital signal unless the traces/cables are several meters or more.  It's ohms law:  voltage drop = current * resistance.  And the current just isn't big enough.
Knowing about and controlling trace/wire impedance is super important.  If you don't know the impedance then you cannot get proper SI.
Using the correct type of signal termination is essential.  I say this again:  ESSENTIAL.
Signal topology (how the signal is routed) is critical in knowing what type of signal termination to use.
Faster clock/signal rates does not determine if a signal needs termination-- it is the speed of the edges of the signal.  Even a reset line that switches only once could still require proper signal termination.
The stackup of the PCB (how the layers are arranged, and the space between layers) is critical for controlling trace impedance.
Pinouts of cables and connectors is critical.  Arrange the signals and power/gnds wrong and it will be terrible.
Decoupling caps are for much more than just making the power smooth.  Again, this is critical.
Higher signaling voltages helps some things, but hurts others.  Unless you are going long distances (more than 10 meters), don't bother.  For example, don't run RS-232 levels if you are just going to the next board in your stack.


I also want to stress again that most things you find on the net get some or all of this wrong.  Arduino stuff especially.  Get a good book, and use that as a solid foundation.  Oh, most books have major errors too.  The book I mention is very good and is considered by almost everyone to be the golden standard.