Although people sometimes use "like" and "as if" interchangeably, there is a correct usage for each. Use "like" as a preposition to point out ideas of comparison or similarity. Prepositions are used to indicate the relationship of one word to another. Other prepositions include as, at, before, by, near and with. Use "as if" as a conjunction to introduce a clause; a clause is a group of related words that contain a subject and a verb. "As if" should be used to explain how something comes across, given the information provided.
Using "Like" as a Preposition
Do not confuse "like" as a verb with "like" as a preposition. When used as a verb, "like" is the action word, as in: "I like potatoes more than rice." When used as a preposition to indicate a preference, another verb is present, as in, "I don't like driving on snowy mountain passes." In this sentence, "driving" is the verb. In the following sentence, the preposition "like" compares two things: "My dog's breath smells like bacon." The writer is using like to compare her "dog's breath" and "bacon." By contrast, in this sentence, the author is using like to compare two places: "When I visited Wales, it looked a lot like Montana to me."
Using "As if" as a Conjunction
While informal speech allows you to interchange "like" for "as if" as a conjunction, formal speech requires "as if." Consider the following sentence: "When he spilled his coffee, my father acted as if the world had come to an end." Whereas you use like to make a direct comparison, use "as if" for the subjunctive -- a statement that is contrary to the facts. The world isn't really going to end over a cup of spilled coffee, so the conjunction "as if" is appropriate here.