Best practice in transmitting sensor analog signal over longer distances (approx. 100') -- in cost sensitive products for microcontrollers
"Best practice" and "inexpensive" are both subject to opinion.

In all cases, I'd put a really inexpensive little processor at the sensor, and have it talk digital.  The 1950-1975 way to do it would be 4-40mA, or have the remote send a PWM or frequency-modulated signal.


Up to six feet or so, power the remote sensor from +5V (or +3.3V) and run ground-referenced asynchronous serial (UART pins) straight from the sensor to the "main" processor.
Up to sixty feet or so, power the remote sensor from whatever, and regenerate it at the remote.  I'd probably give it +12V for overhead, and use a switching regulator "down there", but you can use a linear regulator.  Use RS-422 with the remote set to just send, or use RS-485 in half-duplex, with some sort of a protocol so that remotes only speak when spoken to.