Buy clips for their winter gloves so you don't loose them or sew a string on the ones you have and run it the length of INSIDE the sleeves/arms of the jacket.
Buy labels with your LAST NAME on them (irononcamplabels.com). Then you can simply iron them into EVERYTHING (and use them for all your kids). Use a permanent marker for everything else.
Don't buy a "cute" backpack. They fall apart and don't hold enough. Buy a good one and they can use it multiple years.
See if you can have playdates over the summer with other incoming kindergarteners - esp if you find out who will be in your child's class. Then everything won't be NEW.
Make sure your child can dress herself, pull up her pants (no overalls or tricky buttons on pants). We used lots of leggings and sweatpants for K. Keep shoes simple and durable - no flip flops, sandals that come off, etc. Use velcro gym shoes if needbe - she can wear the cute stuff when she gets home.
JOIN GIRL SCOUTS! Great way to have "instant friends" and fun age appropriate things to do with schoolmates outside of school.
Make friends with the other moms. We always help each other carpooling, taking the kids off your hands when you have a drs appt, etc.
Use the cut apart ice packs. We got ours at Target. They are basically reusable individual packs that are all sealed together but can be cut apart to different sizes. It's a great way to keep their juice box cold.
One thing I didn't learn until later but has REALLY helped:
Change their clothes immediately when they get home - esp during cold/flu season. We have "lounge wear" the kids changed into - sweatsuits or comfy yoga pants and top. My kids come home, empty their bags, wash hands, and change clothes. Dirty clothes go directly into the laundry.
I really feel this has been invaluable in cutting down on germs IN the house and not getting EVERYONE sick.
The drs and nurses during a hospital stay told us that one. They said THEY change BEFORE they go home for just that reason. Keep the germs out of the house and everyone stays healthier.
Believe me, Kindergarten is a big sickness year. It's a combo of being exposed to all those elementary school kids and bad hygiene in most little kids (not covering coughs, picking noses, not washing hands enough, etc). Don't be surprised with all the colds your child gets. It's just part of life.