The Three Shmirois that are in your hands
There are three areas of Tahara that we need to work on.
1) guarding the mouth
2) guarding the eyes and
3) guarding the bris.
The Chida says this is hinted to in the posuk "Odom le'amal yulad." "Amal" stands for Ayin, Ma'or, Lashon. It says in Seforim Hakdoshim that the main effort needs to be concentrated on what is more in our hands, i.e. the eyes and the mouth, and then Hashem will take care of the rest. But the y"h tries to convince us to invest our efforts on what is not in our hands so that we'll end up getting nowhere and being full of frustration.
The Mitteler Rebbe of Chabad explains the posuk "Evilim miderch pishom umei'ovonoseihem yisanu": the way they fell and became evilim was because of "Derech Pishom" -they weren't careful with the causes (i.e. they didn't watch their eyes and mouth). But - umei'ovonoseihem yisanu - instead of doing teshuva and rectifying the causes, they foolishly concentrate on the sins themselves.
This is a very very important yesod for anyone that wants to succeed in any area. Always concentrate on what's in your hands "Kol mah she'yeish bikochachoa asei..."