I use either baking soda or dish soap. Yes dish soap. Shampoo and dish soap are very similar, but dish soap is more designed to cut grease and I always have some on hand. Just don't use very much.

Here's how I use baking soda: dissolve 1 tsp baking soda in 1 cup hot water. Pour over scalp and hair, working it in as I go. Rinse well. Then mix 1 tsp vinegar (any kind, or lemon juice) in 1 cup water and rinse my hair with it to restore my hair's pH.