People should vote because they believe it's making a difference, and they're making a positive statement that they believe a particular person/party will do a decent job of representing them.
The IPPR have suggested forcing 18-year-olds to vote. Because voting levels are dropping, and political engagement is a good thing.
To which my reply is - if you want people to vote, give them a clear choice. Give them a vote for a party that they feel represents them - a vote which will actually contribute towards the success of that party (even in a small way).
Because one of the things I learned from the Independence Referendum is that when you give people a clear choice, where their vote makes a difference, turnout hits 85%.
(It's not entirely surprising that countries with some form of PR tend to have higher turnouts than countries with FPTP.)
Original post on Dreamwidth - there are comments there.