Create a startup.m file in your MATLAB user folder(usually Documents/MATLAB) then useaddpath in that file.
MATLAB will execute that file everytime it starts up, meaning the paths you specify in that file will be "permenantly" available in any folder for all intents and purposes. (At least for all cases someone asking this question will likely encounter.)
One way is to put any such functions into a particular directory (e.g. "utilities"), and then add that directory to the search path using theaddpath command. I put the addpath command in my startup.m file, so it gets called every time I start MATLAB.