If you don't vote, you are implicitly supporting the party you like least, the one you would NEVER EVER vote.
Suppose there are three different political parties: A, B and C. You have no preference between parties A and B (which could be, for example, Democrats and Republicans), so you decide not to vote. But party C is a radical Nazi party that you wouldn't vote for a million dollars (for example, because you're an orthodox Jew and they killed your little dog). Then, if you don't vote, you're supporting party C!
This may seem stupid or exaggerated, but it's the plain truth, and there's no way to get rid of it. Before you decided not to vote, you had two options: to vote or not to vote. If you had decided to vote, you would obviously have voted party A or B, and the value of each vote (at least in most democracies worldwide) is inversely proportional to the number of people who votes, so you would have decreased the proportion of votes for party C. So, if you decide not to vote, you're renouncing to do this, and are thus giving an "ethereal vote" to party C. An ethereal vote is to a vote what a hole is to an electron, and holes are powerful! (see semiconductor)
So... what to do? Throw dice and vote A or B. But there's still a problem. What can you do if you don't want to vote in order to show your disagreement with the system? This is a tricky problem that has no good solution in current democracies, because if you don't vote you're still supporting party C. In my humble opinion, empty seats should be left at the Chambers for people who do not want to vote. That way, people who didn't vote would also decide - and would not be implicitly voting party C.