Google says "do no evil". What exactly does that mean? "Good" and "evil" are highly subjective terms. How should one live their lives by that credo? What is a reasonable standard by which one's actions can judged to be good given the awesome complexity of our interactions with the environment? How do you deal with even simple complex questions like: Is buying a leather shoe, or eating a hamburger bad if you happen to think not killing cows for human convenience is good?
I've found a satisfactory answer to the question above that works for me personally. I'd say a "reasonable" position is one where you give more than you consume. Suppose some action (A) results in some subjective harm (B) through a series of complex interactions (like eating a hamburger which has resulted in the death of a cow). Now, as a result of you doing action A, some other series of events (C) happens which result in a more subjective good (D) (for example, that more cows than the number of burgers you've eaten end up being alive). Then I'd say your action (A) is reasonable and you've done "good".
Now you just have to average and integrate over your lifetime and then see whether the reasonableness standard above applies. I think humans generally are designed in a flawed manner who end up consuming a lot more than they give. So a lot of the elements of the design have to change in the future. Working on that aspect itself and making progress towards it is a way of giving back. People do give back consciously (like scientists trying to invent ways to curb pollution) and unconsciously (farmers who feed the scientists and janitors who clean their offices).
Regarding defining what "good" and "harm" in the first place is also a difficult problem. I primarily use the golden rule and its negation to figure this out, usually putting myself in the place of the subject being evaluated. For example, would I mind/accept being eaten to ensure the survival of another species? The application of this rule also prevents the overuse of "ends justifies the means" actions. So while I may be okay with eating a hamburger to satisfy my hunger through the tortured analysis above, I won't be okay with killing another human being to ensure my own survival.
Overall, I apply the analysis above to every action I undertake and while I also have other ethical standards, this is a guiding principle of the overall path my life has taken.