Honestly can't say for Wisconsin, but you could try planting a hardy cover crop, like maybe winter rye or something. You'd have to check your state extension service for what will last through your winters, but a cover crop is always a good idea. You turn it under at least 2 wks. before you want to plant in spring, and it adds organic matter and nutrition to your soil.
Fall is the best time to prepare a new bed, as the soil is still warm and not too soggy, like it would be in spring. Plus it gives whatever fertilizers and amendments you add time to thoroughly incorporate before it freezes.
The cover crop, btw, will also help keep the snow from compacting your soil, if it does. Having done most of my gardening in TX, I don't have much experience with snow...LOL...

Well, in my experience, it's best to do sort of a lasagna bed on top of your existing one. Do a layer of leaves about 4 inches thick, then on top of that do a 4 inch layer of grass clippings, spent flowers, vegetable stalks, basically green stuff, then a layer of leaves, then more greens... I would only go 4 layers high if you're doing 4 inches thick, but that's what has worked for me.

As for the snow compacting your soil, it does, to some degree, but before you plant, I assume you will be digging it up rather than placing seeds directly into the soil. With the "lasagna" method that I mentioned above, you'll have to mix the compost into the soil before you plant, so any compacted soil will be broken up.

KC

I believe it is better to leave the garden bare, free of any debris. The winter weather will kill many over-wintering insects, weed seeds, bad microbes and bad fungi. No cover crops and do the composting off-site, to be returned in the spring. The freezing and thawing cycle in spring will loosen the bedding for the plant roots to go wherever they want. Where does Mother Nature fluff up the soil before she plants her seeds?

John

Here is a link that might be useful: John's Journal

Well, I'm still undecided as to my approach. First frost has come and gone (came at the 90% chance date given by my sources), and the weather has now given way to snow. The tomatoes and basil got the worst of it and have succumbed; some fruit still ripens on the vines beneath light snow caps. I may still collect some fallen tree leaves to preserve my strawberry crowns, parsley, and a few other mostly herbal perennials, but otherwise I will probably leave the space fairly bare and see if the winter can do its worst. I've got the seeds to start over, if I need to. (I have a seed-buying problem -- or advantage, if you might agree.)

Why will I take the low-key approach? Keeping my strawberries apart was a mistake; they all grew at awkward intervals and have a better chance to survival in a cluster. I had poor defenses against pests and they were eaten despite my best climatic efforts. My thyme and marjoram were overcome by tomato shade this year and need transplantation or replacement anyway. I expanded my square-foot-garden (to be later) filled with Mel's Mix by over sixty percent, so I will have more opportunities next season. Until then, I have a lovely whole in the ground to remind me when the weather was hospitable.

As for materials, the late summer here was so dry and cool that we last mowed in July and coasted until first snowfall. The green leaves have now shriveled and fallen into the few yellows we have. No freshly-mown grass, some leaves, but the oaks haven't made their moves yet. I will probably gather some leaves and put them in the square-foot-garden, if only for the high nitrogen content they will slowly release. I already know my home garden will be far more focused on salad next year than fruit; will be shade-tolerant; will be deer-proof. In that mindset, closing up is as easy as letting the snow fall. Next spring will afford countless more opportunities to fix the mistakes I see in hindsight now that it's cold.

Thanks to everyone for weighing in, and for your expertise.