To improve your spelling, keep studying the most difficult words, sound each word out loud, and know the rules of suffixes and prefixes. Read often, as it helps to see how words are spelled out to eventually remember the spelling.
A good way to start improving spelling is by making a list of words that are commonly misspelled and practicing them every day. Start by studying the word's spelling, and then write it down 10 times. Keep doing this every day until the difficult words become easier to spell. Another way to find words commonly misspelled is by keeping track of words that often require correcting through spellcheckers.
Another good method for practicing a word's spelling is by saying it out loud and them spelling it out loud, one letter at a time. For some people, actually saying it and hearing it is more effective than just writing it.
Learn what prefixes and suffixes are and how to use them properly. This helps for words that are often mixed up with similar words because they are spelled similarly. Furthermore, look at common root words, as these are often used in numerous other offshoot or related words in the English language.
Spelling programs such as Ultimate Spelling can also be a helpful tool. The programs include practice and games to help you learn lists of words that are difficult for you.