Correctly using like and as will let your discerning readers or listeners know that you know. It will also make your message clearer.
Good instruction in this can be found in The New York Times' blog on the language, After Deadline. Philip Corbett writes:
The word like plays many grammatical roles. The one that raises a usage issue is its sense as a preposition meaning similar to. In that guise it can introduce only a noun or a pronoun: He deals cards like a riverboat gambler. If in doubt about the fitness of a construction with like, mentally test a substitute preposition (with, for example): He deals cards with a riverboat gambler. If the resulting sentence is coherent, like is properly used.
But when like is used to introduce a full clause — consisting of subject and verb — it stops being a preposition and becomes a conjunction. Traditional usage, preferred by The Times, does not accept that construction:He is competitive, like his father was. Make it as his father was, or simply like his father. If the as construction (although correct) sounds stiff or awkward, try the wayinstead: He is competitive, the way his father was.
In other cases, if like fails the preposition test, as if may be needed: She pedaled as if [not like] her life depended on it.
When like is used correctly as a preposition, it faces another test. The items linked by like must be parallel, and therefore comparable. Do not write Like Houston, August in New York is humid. That sentence compares August to Houston, not what its author meant. Make it Like Houston, New York is humid in August.Like, you know?