In and of itself, simply telling people not to vote is will lead nowhere. If you're inclined to vote, you see an important difference between candidates/parties. For many Americans, that's the case. And it's true: democrats and republicans have different policy positions on a number of issues. However -- and this is especially damning in the case of all the voting leftists who are conscious (in varying degrees!) of the evils of capitalism -- these differences are very minor once you take a cliched "look at the big picture."
Example: for many liberals and leftists, the war against Iraqis is the greatest evil today. The 2000 and 2004 elections, then, were extremely important. They decided whether the war happened at all and (supposedly) what course it was to take. For communists, however, who know the scales of human misery, the war is insignificant; more people starve to death in three days (because of capitalist production) than die in Iraq in three years of war. Until abolishing capitalism is on the ballot, elections are important only to people who accept capitalism as legitimate.
Unless it is part of a larger offensive against capitalism's ideological dictatorship, unless we challenge the very legitimacy of capitalism itself, anti-electoral propaganda will fall on deaf ears. However, we can use elections as an opportunity to show that regardless of whatever differences exist between competing candidates and parties, all politicians are in thrall to the logic of capitalism. For instance, look at what happened when the Socialist Mitterrand was elected in France. He set about effecting the changes his leftist electorate demanded, but before long he was in charge of a state that was following the same path as Reagan and Thatcher. We can also point out that the majority of American proletarians have already concluded -- quite wisely, too! -- that voting for capitalist politicians isn't worth giving up an hour of free time.
And you can't eat your cake and have it too! You cannot remain fundamentally opposed to capitalism while retreating from the terrain of class war to vote for some capitalist offers this or that palliative. Legitimating the state in exchange for some meaningless reform means accepting capitalism as a valid system with just a few problems, and ones that can be resolved at that!