Edit: Immediately after posting this, I discovered that the Lib Dems have ruled out the possibility of a deal with Labour. That being the case, you can downgrade "one chance" to "no chance", and forget the advice below on how to vote - we're getting the Tories regardless.
In any* UK General Election, there are only two possible (final) outcomes: either we get a Tory-led government, or we get a Labour-led government. It's also just possible that we get a situation where nobody can form a government, but that just results in us having to vote again.
(* Okay, that's the current paradigm. Long-term, there's the possibility that one of the big two parties will collapse so completely that they are overtaken by a different national party. In which the same "three outcomes" apply, just with one of the names changed. For an example of this happening, see the rise of Labour at the expense of the old Liberal (not Lib Dem) party.)
So, this election boils down to a choice: which is it to be - Tory or Labour? They both suck, but they're the only options, so when it really comes down to it and you have to choose, what's it to be?
Now, if you chose Tory, here's the good news: you're almost certainly going to get what you want. I suggest you go out and vote Tory, though you do have a few other options available.
But if you chose Labour, then the news is nothing but bad. Firstly, because you're almost certainly not going to get your wish, but also because you're probably going to have to do something you really don't want to do.
Here's the thing: to have any chance of a Labour government, we really need to maximise the number of non-Tory MPs in the House of Commons. When push comes to shove, the Greens, SNP, Plaid Cymru, and similar will vote to support a Labour PM but not a Tory one, while the Lib Dems will probably join such an alliance if the numbers are otherwise there. And when push comes to shove, Labour will probably accept that support in order to keep the Tories out. (Besides, we might as well assume that that's the case - if it's not, we might as well not bother voting.)
So, since getting the Tories out means voting as many non-Tory MPs as possible in, here's how it needs to work:
If you live in a Tory-held constituency (or Douglas Carswell's constituency), then you need to go and vote for whichever party came second in 2015. Unless that was UKIP (or the Tories, in Clackton), in which case go for the third party. If you live in a constituency that is not Tory-held (or, again, Douglas Carswell's seat), then you need to go and vote for the party that won the seat last time.
Now, this does mean that a lot of people will have to vote for people and parties they really don't like - it means lots of traditional Labour voters in Scotland voting SNP, it means a lot of people who detest Jeremy Corbyn going and voting for him, and it means that the people who gave the Lib Dems a well-deserved kicking in 2015 voting them back in. I'm sure none of that appeals.
(It also means that, in the very best case, we get a government formed from a crazy coalition who don't agree about almost anything, led by a Labour party in disarray, and with the Prime Minister who can't even command the support of a majority of his own MPs. Which I would normally consider a profoundly irresponsible thing to vote for, except that I'm convinced that if the Tories win then England and Wales are headed for economic catastrophe while Scotland and, probably, Northern Ireland are headed for the exit door.)
But that's the price. The margins we're working with are so small, and the polling situation so desperate, that even a mass tactical vote is highly unlikely to be enough to swing this one; without that mass tactical vote, there's no chance at all.
(No, really. The most recent poll of voting intentions put the Tories in 48%. That's already an almost unbeatable position - blocking them requires the rest of the vote to line up just right.)
Given the position that we are in, a vote for anybody else, or even an abstention, is effectively a vote for the Tories.