When you're done writing your story (or essay or paper or whatever), run it through a text-to-speech programme (the tip suggested Google Translate, which will read what you paste into it aloud if you press the speaker button, though make sure you're on English). You will definitely hear the mistakes, as, because you know what it's supposed to say, your mind fills in the gaps when you read it to yourself, but hearing it read aloud by something that doesn't know what it's supposed to say makes you pick up on all the missing words or wrong tenses, which is something I have a lot of trouble with.
It's also hilarious to hear a computer try to suss out your character names. My favourite was one UK Speech Programme which for some reason registered 'Siger' as something closer to 'Sigula'. And hearing the unemotional computer voice reading out your characters being emotional is also quite entertaining.
ETA: OMG, I got an extra birthday present today, Rainbow Dancing Water Speakers, and I just hooked them up and OMG, they are the best thing I have ever seen. Seriously. My speakers are dancing.
I am so easily entertained.