I think the best policy is assertiveness, dealing with injustices or exploitations as they arise, and face to face - not later or in a revengefull way. If we think someone treated us unfairly, then we should say that or prevent them doing that - we should do it in a firm but polite manner, or if we see it´s too much or the situation is hopeless, leave.
How else do you think injustice in the world can be eliminated, than being just yourself? You can also be a teacher and try to bring up moral values in children, best serving as your own example.
You can be a humanitarian worker, but that won´t change the roots of this evil. If you give man a fish he will have food for one day, but if you teach him how to catch fish he will have food for the whole life.. Just giving people things will alleviate them for a bit and enables them to survive, but won´t solve the problem of world injustice imo.
You can be a political activist trying to fight against establishment, of course. But as everyone who gets to the top gets corrupted, I don´t have much hopes that there will ever be justice in the world, if you look into history, people were always exploited by those on top. Why, is every single person corruptable? At least you can make sure you aren´t.. Everyone should make sure they are completely honest themselves before going to fight against some injustice in the world, I think. Or do they think their small injustices don´t matter? They DO!
Then there are injustices and unfairness in people´s personal lives, like someone offends or betrays us etc. I think we should deal with this with assertiveness too, then move on and don´t hold the grudge. I have seen ppl who carry their grudges for their whole lives and are unable to forgive, still going on and on how someone did them wrong. I think it is normal to feel hurt and angry for a while, but inability to let go and forgive seems rather like they cannot get over their own ego/hurt pride.