Personification, apostrophe, and imagery are the main techniques used to employ meaning in "To Autumn." Namely, Keats uses personification in order to give Autumn human qualities in almost every single image. The most famous one from the poem, of course, is in calling Autumn the "close bosom-friend of the maturing sun." Autumn is also shown to be "conspiring" with the sun in order to produce a fruitful harvest. Therefore, the sun is also personified indirectly (in that it is a "friend" and a "conspirerer" as well). Autumn is also described as "sitting careless" and having "hair soft-lifted" in drowsing.
Keats also uses apostrophe in his poem to help employ meaning to the reader. Apostrophe is the device used when a poet invokes something that is not human (an animal, an idea, even a dead person) or someone who is not there with direct address. Keats directly addresses Autumn mostly in the last stanza of his poem after he questions where the "songs of Spring" are by saying "Think not of them, thou hast thy music too." Sometimes apostrophe can do more for giving an abstract idea human qualities than even personification can!
Of course, no one could talk about "To Autumn" without mentioning the rich imagery here! All five senses are evoked! In regards to sound images (which are mostly represented in the last stanza), we have the buzzing "bees" and the "winnowing wind" and the "music" of Autumn as well as the "choirs of gnats," the "lambs loud bleat," the songs of "Hedge-crickets," and the "red-breast whistles." There are plenty of touch images as well such as the "mists," the "clammy cells" of the bees, Autumn's "soft-lifted" hair, the "oozings" of the ripe fruit. Touch, of course, can bleed into taste imagery as the "oozings" of ripe fruit also appeals to taste as does the "fruit with ripeness to the core," the "sweet kernel," the "cider press," and simply the plural noun "apples." In regards to smell (the least used method of imagery here), Keats adds "later flowers for the bees" and "the fume of poppies." In regards to sight images, most every noun can be one. Most of the examples above can also be sight images. However here are two of my favorite collections from the poem:
To bend with apples the mossed cottage-trees, / And fill all fruit with ripeness to the core; / To swell the gourd, and plump the hazel shells / With a sweet kernel; / to set budding more, / And still more , later flowers for the bees.
Full-grown lambs loud bleat from hilly bourn; / Hedge-crickets sing; and now with treble soft / The red-breast whistles from a garden croft; / And gathering swallows twitter in the skies.
Yes, Keats' poem with all of it's glorious imagery makes me wish for sweet autumn to be here (instead of this horribly chilly and pollen-filled spring).