6 And God said unto him in a dream, Yea, I know that thou didst this in the integrity of thy heart; for I also withheld thee from sinning against me: therefore suffered I thee not to touch her.
Obviously we should all do our utmost to preserve good and prevent evil. This verse gives an important reminder that God will be championing these same causes as well, independent of us. Things were looking like they might have gone a bad way for Abimelech, Sarah, and Abraham, but God intervened and did not let that happen.
I have had times in my own life where God has intervened to prevent me from evil, He has even saved me from my own willful foolishness. I have also had times where I wanted to help another, but had no way to do so, and God assured me that He also wanted that person to be helped, in fact He wanted it even more than I did, and He would take care of it.
So yes, we should do what every good thing that we can, but the good that we cannot do we should surrender to God, trusting that He is capable of bringing good and preventing evil all by Himself.