A few simple things to remember when proofreading your work.
1. The basic read through.
Read through your work at least once to get an overall feel for the story, characters, and dialogue, before you proofread for particular errors.
2. Proofread the text.
Next up should be punctuation, grammar and spelling irregularities (including the consistent use of alternative spellings based on any style guide used by a company). This is of particular importance if you are a British writer submitting material to US-based publications and vice-versa.
3. Read out loud.
Another great way to check your work is to read it out loud, especially when checking character dialogue. Try have another person listen or read along with you. This is also a good way to catch missing or repeated words.
4. Double-check names and numbers.
Check the spelling of character and place names throughout, this is important if you’ve created a new language.
5. Style and consistency.
Don’t forget when and where to use italics for character thoughts or for highlighting exposition in flashback.
6. Proof from a printed source.
Where possible always proofread from a hard copy rather than relying on your word-processor’s spell-checker, you can also make notes as you read.
7. Beta Readers.
Also, try and find another person/writing partner who is willing to Beta Read your work for you.
8. Writing Groups.
Finally, there is no better way to know if you are on the right track than working with a bunch of fellow writers for help and feedback. Think of joining one and gain yourself a set of potential beta-readers.