Always be sure you're using the right word. If you're not sure, 
don't use it! Here's a great list from Woe Is I.
nauseated/nauseous. It's the difference between being sick and sickening. You are made sick (nauseated) by something sickening (nauseous). Never say, "I'm nauseous." Even if it is true, it's not something you should admit. "I'm nauseated by that nauseous cigar!" said Ethel.
on to/onto. If you mean on top of or aware of, use onto. (The responsibility shifted onto Milo's shoulders. I'm really onto your shenanigans," he said.) Otherwise, use on to: Hang on to your hat. Sometimes it helps to imagine a word like "ahead" or "along" between them. Milo drove on to Chicago. He was moving on to better things.
different from/different than. What's the difference? The simple answer is that different from is almost always right, and different than is almost always wrong. But... You may use either one just before a clause (a group of words with its own subject and verb). Both of these are accepted: Respectability is different from what it was fifty years ago. Respectability is different than it was fifty years ago.
continually/continuously. Yes, there is a slight difference, although most people (and even many dictionaries) treat them the same. Continually means repeatedly, with breaks in between. Continuously means without interruption, in an unbroken stream. Heidi has to wind the cuckoo clock continually to keep it running continuously. (If it's important to emphasize the distinction, it's probably better to use periodically or intermittently instead of continually to describe something that starts and stops.) The same distinction, by the way, applies to continual and continuous, the adjective forms.
farther/further. Use farther when referring to physical distance; use further to refer to abstract ideas or to indicate a greater extent or degree. Lumpy insisted that he could walk no farther, and he refused to discuss it further.
Source: Woe Is I, by Patricia T. O'Conner.
