"Wear old, warm clothes. Gloves, rubber boots, and raingear are strongly encouraged." Really though, whoever you call up to volunteer with, ask them for advice on how to dress, right down to recommended brands if they will. It's a sure shot that there won't be any available where you volunteer, so best if you can buy your Carhart's or other gear where you're at.

City of Fargo has up a flyer on building a sand bag dyke, see http://www.cityoffargo.com/Flood09/Sandbagging/.