Stealth and don't kill anyone, only knock out or tranquillize, that will give you S ranks, also try to get in some tactical takedowns.
-mostly what's above
-no resets
-no chopper support
-no camo, chicken hat, parasite armor,... many items prevent S rank
-no kills, no detection, no slowmo mode.
Go Fast, the quicker you are, the more mass points you get.
...it's not only the points. Check your equipment whether something is flagged 'restricted to A rank and below'.