I didn't vote in the recent California primary. I could site minor excuses (like I didn't get around to registering locally in time), but basically, I tend to be apathetic about voting.
I think the most meaningful function voting might serve is, no matter who wins, the votes get counted and they send a message to politicians (but especially to those who can change their principles at a moment's notice). Thus voting for Micky Mouse or other such candidates isn't valid because few people will ever know about it.
The reasons for not voting are: 1. Whichever way I vote, on the very slim chance that it gets someone elected or a proposition passed, it's unlikely to really change anything. Few arguments in a two-sided debate address what I feel needs to be done. Perhaps that's a major reason I started this web site. 2. If a candidate or issue wins by more than 2%, it's supporters tend to take it as a mandate from the people and try to go further with it than the people wanted. The next election, the pendulum swings back again. That's partly why we need more than a two-party system.
It would also be good to have at least a guideline that variable economic factors shouldn't be changed by more than 5% per year. Thus the Democrats get in power and pass a law to increase social services by 5% per year for 20 years. After three elections, the people decide it's too much and vote in the Republicans, who in turn create similar decreases, and are voted out after two elections. The conditions never change too far from what the people think is optimum.