Preparing a 100 Ohm resistor for use as a PT100 standard
How do you mount it?
Should be done without physical stress. This is both during any lead bending (even then check the component manufacturer's guidance on how close to bend to the body) and in service. 

I specified bent loops for a Standards compliance machine design: 
through hole - loop -- body -- loop -- through hole
Then assemble and check it works, finally throw it in the corner for 6 weeks to age. (in practice we found it made little if any difference whether kit was powered up, subtleties here too, some components age exponentially from last curie point)