"Like" versus "as"

As far as the first two sentences go, they mean the same things. However, a couple points of usage:

Dress like if you were having guests.

If is not necessary in this sentence. The word like here means similar, so it is not a conditional, but rather a comparison. The original sentence uses if because of as—how should you dress? As someone expecting to have guests = as if you were to have guests. Also, the original sentence might be more succinctly found as "Dress as if you were having guests".

She kissed him goodbye, like the usual.

The is not necessary, like the original sentence.

Sweet like she is, she doesn't pay his bills.

This construction is not found among native English speakers, who always use the word as. The reason is this: like expresses similarity, but in this context, as expresses quantity; in other words, the sentence is saying that she is very sweet. The altered sentence, however, is meaningless.

"Like" is a preposition; "as" is a conjunction. So it is right to say, "she kissed him goodbye, as usual." "Dress as you would if you were having guests" is correct, but it might sound better to say, "dress as though you were having guests."

However, "like" is commonly used as a conjunction, and only grammarians will notice if you say, "she kissed him goodbye, like usual." "She was like my mother" is correct. "Like" means similar to, so "like usual" doesn't make much sense; it signifies that she "kissed him goodbye, similar to usual." But the sentence wants "to indicate by comparison the way that something happens or is done" (New Oxford American Dictionary).