memory usage "dos and don'ts"
As you have noticed yourself, this can be somewhat platform-dependent.
Since you are working on a Nano, my answer will be for the AVR
architecture, when compiled with gcc, the avr-libc, and the Arduino core
library.

First, let me reassure you: this weird behavior you saw of the Wixel
doesn't happen on this platform. Local variables get allocated mostly in
CPU registers, then on the stack.

Now, the first general rule may sound obvious: think twice about what
your program really needs to remember. Also, use each time the
smallest possible data type. The C99 types int8_t, uint16_t and co.
are useful for this.

Another rule you will often hear on this platform: avoid dynamic
allocation, i.e. malloc() and new, as quite often you cannot afford
heap fragmentation. Avoid also the String class, as it relies on
dynamic allocation. Prefer when possible static allocation, or stack
allocation for small things.

Don't forget to use const for constants: with this qualifier the
compiler can often optimize the constants out as immediate operands. For
arrays of constants, use PROGMEM. Yes, it's not comfortable
to use, but it can save you lots of RAM. The F() macro for printing
constant messages is a special case of PROGMEM.

Lastly, be very careful with recursion.