Say the word out loud. Use phonetics and pronunciation to try and build the correct spelling.
Look for similar patterns in words that you know how to spell. Memorize consonant combinations, suffixes, prefixes and vowel groupings. Use root words to build the correct spelling for different words.
Use mnemonics like "'i' before 'e' except after 'c.'" Come up with simple rhymes to help remember spelling rules.
Write a questionable word out several times to use your visual memory to see which version looks right.
Read the text out loud when proofreading. Listen to the flow and look out for grammar mistakes. Read out loud in your head if you are in a setting where you are unable to vocalize.
Double-check homophones and problem words. Simple mistakes with words that sound the same but are spelled differently can be easily passed over.
Read slowly, word by word. Attention to detail is essential in proofreading and rushing through it will cause errors.
Read frequently. Reading not only gives you a visual frame of reference for correct writing, but it helps you become a better writer.