This is really simple:
Suppose you want to check if some action of yours makes a difference.
How to do it?
The wrong thing to do:
Think of the consequences of your action and evaluate them to see if they fit your purposes. If they do, go on and do it.

The reason for this being wrong: If someone else does something with the same consequences, and if your doing or not your action makes no difference to the fact that THAT person will do it, then you are not necessary for those consequences, they would happen anyway.
This is also true if something, not a person, would do an action with the same consequences.

The right thing to do: Consider what would happen if you DIDN’T do your action. Subtract that from what would happen if you DID do your action.
This is the difference it would make if you did it.

There is a reason it is called a ‘difference’, it is the difference between you doing it and you not doing it.

Example: Suppose you think you will make a difference by carefully considering your vote, and voting.

Wrong: Well, I’m partially causally responsible for the election of X so my action would make a difference.

Right: If I do vote or if I don’t vote, the same candidates will be elected. Therefore my vote makes no difference.
(In more than 16000 elections in the USA it was NEVER the case that one vote would have made a difference)

The AWFUL argument people usually say: But what if everyone did it?

The reason it does not work: Everyone will NOT do it. Yes. That simple.

The reason it is awful: Compare “I don’t think I should go to the movies today, what if everione did it?”

So, when you are willing to make a difference, not feel good, not do what everyone does, not clean your consciousness. When you want to REALLY, REALLY make a difference, you should consider the difference between doing and not doing it.

It is that simple.

(Note to the Less Wrong entry: I know most people here know that politics is a mind killer, but delving deeper into the simple argument, notice it works for any case of overdetermination, such as posting a comment or entry that someone else would. If you realize now how important it is to figure out what is important, don't forget more than one person have already done it, use them.)