Think about the meaning of the word.
Is it related to another word you know and can spell?
If you can spell one word, a related word will probably be spelled similarly.
Use a dictionary.
If you can't find the word you want in your dictionary, click Pronunciation→Spelling for some help.
It is also worth learning the system used for showing pronunciation.
Use your computer spell-checker actively. Don't just accept what the computer suggests - computers are often wrong. Make sure it is correct. If it is a word you know you cannot spell, write it down and remember it.