10 Words to Avoid in Formal Writing:

Purists may tell you that many of the words in the list below aren't "really" words at all, but that's misleading at best. A few of the words are simply misspellings, and the rest often appear in people's everyday speech (or vernacular).

Nevertheless, according to the conventions of Standard English, all 10 words should generally be avoided in reports, essays, research papers, and other kinds of formal writing.

1. alot
   Alot (one word) is a common misspelling of a lot (two words). "[W]e all may write alot one day," says The American Heritage Guide to Contemporary Usage (2005), but for now "keep in mind that alot is still considered an error in print." 
2. and etc.
   Because the abbreviation etc. (from the Latin et cetera) means "and so on," and etc. is redundant. In any case, avoid using etc. in your essays: often it gives the impression that you simply can't think of anything else to add to a list.
3. anywheres
   Huck Finn can get away with saying, "There warn't a sound anywheres," but on formal occasions drop the terminal s. If anywheres appears anywhere in your dictionary, it's probably labeled "nonstandard" or "dialectal."
4. could of
   Don't confuse this nonstandard form with the contraction could've. Could of(along with should of and would of) can and should be replaced by could have(and should have and would have). As for coulda, shoulda, woulda, avoid dwelling on them—both in writing and in life.
5. hisself
6. This alternative form of the reflexive pronoun himself is commonly heard in certain dialects, but in formal writing steer clear of hisself (and theirself as well—though both were regarded as good usage in Middle and Early-Modern English).
7. furtherest
8. The comparative form of far is farther or further. The superlative form is farthest or furthest. Nothing's gained by combining the two forms. 
9. irregardless
   This double negative (ir- at the beginning and -less at the end) may not deserve Bryan Garner's label of "semiliterate . . . barbarism," but he's probably right that in print it "should have been stamped out long ago" (Garner's Modern American Usage, 2009). Use regardless instead.
10. its'
    Its is a possessive pronoun (like his or her). It's is a contraction of it is or it has. That leaves nothing for its' to do—so toss it. 
11. let's us
    Let's us means "let us us." To avoid the repetition, write lets ("She lets us play in her yard") or let's ("Let's play in her yard") or let us ("Let us pray"). 
12. nohow
    If you have the know-how to write, you don't need to be told to avoid nohow. Instead use in no way or not at all.