Clothes that layer well, like bicycle shorts for under dresses to cover underwear.
My daughters would were dresses 365 days per year, but sometimes the wether does not permit it - so leggings and plain tees work great under dresses that are too springlike.
A good sweater cartigan to have in the school bag every day in case it gets cold.
Soft pants that allow a child to move, not just jeans, that will be easy to play in and have gym class in.
One great pair of sneakers. All other shoes should have good soles an dnot be slip-ons for running and climbing so they will be safe on teh playground and in PE. Slide on shoes are a bad idea. Also in our preschool, they don't allow boots (rubbers or snow, because of the tripping hazard. They do allow them at the elemebtary school. However, teachers don't want to be helping 25 kids take their boots or "rubbers" on and off several times per day, In first grade, my daughter couldn't even change shoes for PE, she had to wear sneakers that day. So assume whatever shoes they wear to school are the ones they have on all day, regardless of weather. It's tricky, to get the right collection of shoes that are still pretty and go with everything!
Undershirts! Another good layering idea for warmth. And a pair of "long johns" or winter underwear.
Try to to pick everything in 2-3 coordinating colors, so you can match no matter what. For instance, I always buy a lot of navy blue, pink, and brown, because I know this well together from fall to spring and coordinate well with different shades.
Look for gloves, hats, and earmuffs too and buy an extra pair to always have in the school bag. A little umbrella too, if they ride the bus. (and a big one for you)
Have fun, and look for a tax free shopping day in your state. In VA, it is the weekend of Aug. 5. Have fun!!