Use a dictionary
Look for correctly spelled words that may in fact be typos
Untied States instead of United States
from instead of form
Choose the right homophone
its vs. it's
effect vs. affect
Use the exact spelling and capitalization for proper nouns
Marian Anderson, not Marion Andersen
HotData, not Hot Data or Hotdata
For more information, see Oxford Dictionaries' Commonly Confused Words and Spelling pages.