1. Use spellcheck (or an outside grammar tool)
This is really the most fundamental thing you need to do before anything else. If you are writing in Word, even running the basic Spelling and Grammar tool can catch a whole host of mistakes that you've made. You can go one step further and use another tool such as Grammarly or Paper Rater too. Make sure you use your best judgment though - automated tools can only go so far, so they will often pick up things that aren't actually errors.
2. Read out loud
You might feel silly, but there is nothing like reading out loud to help you see mistakes like confusing sentences, word duplication or repetitive vocabulary. This is a great way to eliminate careless mistakes and refine your sentence structure.
3. Take a break before rereading
It's easy to get too wrapped up in your work to see things objectively. After your first proofread, take a step back and tuck your manuscript away for a few weeks then come back and reread it. As well as seeing errors that you missed the first time, you might have a few ideas for story tweaks and be better at spotting plot holes or inconsistencies too.
4. Ask for a second opinion
If you're not sure if a word is in the right context or if a sentence sounds good, ask someone else. Forums like KBoards are great for getting a second opinion from other writers - or you could just ask a grammar-savvy friend or two. If in doubt, always check, don't just let things slide.
5. Learn the rules of grammar and punctuation
If you are going to take the time to self-edit your work, make sure you know what to look for. Not sure when to use a comma, or how to punctuate speech? There are hundreds of free online resources to help you out. GrammarBook and Grammar Girl are two handy resources, but just type your query into Google and do a bit of research and you'll be able to learn the rules in a jiffy.
6. Make a list of common mistakes
If you know you are prone to mixing up your and you're or using then instead of than, make a point of checking specifically for those mistakes. Here is a list of common errors to check for - you can use the Find tool to skip through the manuscript to find specific words.
Any more suggestions for self-editing? Write them in the comments below.