As Michael Moore has pointed out, if you want to impact politics, it's pretty simple: go to a local party meeting with 10 friends (all of whom are registered to that party, of course). You'll take over the precinct, and then you can have a real affect on local and regional party politics...if you really want to make a difference, that is.
Not voting is a pretty ineffective way to effect change. I offer the following quotes from Robert Heinlein on voting and democracy:
Democracy is not an automatic condition resulting from laws and constitutions. It is a living, dynamic process which must be worked at by you yourself - or it ceases to be democracy, even if the shell and form remains.
Politics is just a name for the way we get things done... without fighting. We dicker and compromise and everybody thinks he has received a raw deal, but somehow after a tedious amount of talk we come up with some jury-rigged way to do it without getting anybody's head bashed in. That's politics.
One way or another, any government which remains in power is a representative government. If your city government is a crooked machine, then it is because you and your neighbors prefer it that way - prefer it to the effort of running your own affairs. Hitler's government was a popular government; the vast majority of Germans preferred the rule of gangsters to the effort of thinking and doing for themselves. They abdicated their franchise.
If you are part of a society that votes, then do so. There may be no candidates and no measures you want to vote for ... but there are certain to be ones you want to vote against. In case of doubt, vote against. By this rule you will rarely go wrong. If this is too blind for your taste, consult some well-meaning fool (there is always one around) and ask his advice. Then vote the other way. This enables you to be a good citizen (if such is your wish) without spending the enormous amount of time on it that a truly intelligent exercise of franchise requires.