self-balancing

I'd like to start a discussion to collect general knowledge of what to do and what not to do when building a two-wheeled self-balancing robot (SBR for short).  My hope is that this thread can become a somewhat comprehensive set of guidelines to help beginners and non-beginners build their first balancing robot or  achieve a more stable and successful subsequent design.  Things that I would like to understand include:

Making a balancing robot - or even dreaming of making one just lost it's appeal.. because I will never make anything like this.,

I was sent this cool video of a self balancing Robot. My niece wants me to buid one for her.