He uses sound, especially alliteration: dull, dark, dreary, desolate, decayed depression, dreariness--and this is just paragraph one.
He uses punctuation as a rhetorical strategy: in paragraph two the narrator frequently breaks us his speech, which the text signifies through the dash (--) to indicate his fragmented thoughts brought about by what he sees, the House of Usher.
He uses personification, referring to "eye-windows" twice in paragraph one.
He uses sensory images repeatedly to create mood: the day is "soundless," the trunks of the trees are "white," and there is an "iciness" about him.
He uses analogy: he feels a depression somewhat like the "after-dream" of opium; "it is like "the hideous dropping off of the veil."
One for good luck: he uses metaphor in paragraph two when he says "it was the apparent heart of the request."