This election, like every other, but also quite unlike them, is going to come down to small numbers of votes in a small number of seats.
Find out what your constituency is, and who has a realistic shot of winning, just need to enter your postcode about halfway down the page.
If the 'Swing's listed are anywhere near 10% then you can stop worrying and pretty much vote with your heart, because your vote (like so many others) doesn't really matter.
If the 'Swing's are less than 5% then you've got a real chance to decide your MP, but probably only between a couple of parties. At that point, I would suggest voting for the lesser of whichever two evils are presented
EDIT: Unless you're in Scotland, then enjoy the 30% swing to SNP and ride the wave!