Like V. As

For a detailed and technical breakdown of "like" vs. "as," check out Grammar Girl's site.

Notice that like is used to compare two nouns:

Harold's painting looks like a child's experiment with finger paint.

As is used to compare two clauses:

Writing a good essay requires students to concentrate as if they are baking a cake.

One way to test this out is to follow this guideline from Rutgers:

As versus Like.

In formal writing, avoid using like as a conjunction. In other words, something can be like something else (there it's a preposition), but avoid "It tastes good like a cigarette should" — it should be "as a cigarette should." Quickie test: there should be no verb in the phrase right after like. Even in phrases such as "It looks like it's going to rain" or "It sounds like the motor's broken," as if is usually more appropriate than like — again, at least in formal writing. [Entry revised 12 April 2001; moved 10 December 2006.