The Guardian's guide to the practicalities (link courtesy of rydra_wong)
tactical voting guide (of course, you might know better; I disagree with them about Colchester, where I'm far from the only person who will never vote again for the former MP who voted for the Health and Social Care Act 2012).
If you already know who you want to vote for and you're certain they won't win, or that they'll win easily and don't need your vote, vote anyway! On the one hand, several recent elections and referenda prove that nobody knows anything in politics nowadays; on the other, one day electoral reform will be debated in Parliament again and the more egregious the gap between votes and seats is, the more seriously they might take it. If you don't vote, you will be ignored.