Proofread! Proofread! Proofread!

You don't know pain like sending off a piece of work, then finding a typo the second it's sent.
Heartbreaking.

You can get other people to have a read through, or use a spell checker, but unfortunately, some things can be overlooked. I have only sent one essay off with a typo, so far, but I was fortunate to contact immediately and had the chance to rectify the mistake before the submission was accepted and the deadline was still open.

Proofreading seems like an obvious thing to do when writing, but as I said words can be overlooked, especially when you've added an extra word, your spell check may not flag the mistake, and your brain may completely miss it when having a quick skim when writing.
I've learnt to write, edit twice, proofread, and then have beta readers involved to avoid any typos slipping through. It's also a good way to hear your work whilst you're writing it - reading aloud whilst writing has helped me notice the words I've used.

Tip: Writing in comic sans apparently helps you notice mistakes, and makes you write more!