Preparing Beds for Spring Sunflower Planting?

1) Since sunflowers are annuals and will not come back, there is no reason to leave the stalks in the ground over the winter unless you like the appearance or want something for the birds to perch on.
2) Remove all weeds asap so no weed seeds are added to the bed. You can dig/till it now, or, even better, just put down a thick layer of mulch, leaves, straw, etc. to enrich the soil. If you do this, you can just plant through it in the spring. (called "no-till" or lasagna method, let the worms do the work).

3) Crop rotation is usually thought of more with vegetables, but is important whenever you grow plants that are prone to diseases that can remain in the soil. I don't know many flowers that fall into this category. In fact, perennials are grown in one place for years. Do web searches on the plants you plan to grow, with the words "rotation" or "common diseases" and see if you can find out more.