In a democratic election the desired result is for the government to be formed by those for whom the majority of the people in the country vote. The first past he post system destroys that possibility and destroys democracy.
So.....what to do? Look at the past history and decide how to vote - if for example you support Labour policies and the candidate in your constituency stands a chance of getting in, then vote for her. If she doesn't, then what to do? Waste your vote on her or try and make darn sure the Tories do not get in by voting for whoever, in your judgement, stands the best chance of tipping them out and whose policies you find more tolerable.
I live in Tory heartland with just a very brief sojourn by the Lib Dems. In the last election the Tory candidate won by a huge majority, and is likely to do so again. If I vote Labour, it will be to all intents and purposes a vote for the current Tory MP. If I vote Lib Dem, there is a small chance that the current MP will be toppled and reduce the Tory majority, which for me is the desired outcome for all the reasons that others have eloquently described above.
Tactical voting is a way of trying to redress the undemocratic influence of the first past the post system.
Casting a vote that you know will have the opposite result to what you desire is just plain bonkers!