Reduces by 250 each torment level.
Just for easy math:
Current armor is 1000
T1 reduces to 750
T2 reduces to 500
T3 reduces to 250
T4 now at 0
Put armor tempering % on something, use skulls in jewelry to boost your armor. Also put an affix of armor on something too. Use juggernaughts on chest or helm.
Resistances use diamonds in jewelry and find/put resistances on items that you can put them on. Also each torment level you go up, it reduces by 50% each time (veterans, correct me if im wrong, im typing as im tired)