Dashi is one of those pantry staples everyone should have on hand. It freezes very well and lasts indeffiently. The ingredients will probably require a trek to the nearest Asian grocery in most places, but it is worth it. The stock is very light but high in flavor and is the basis for many soups and dishes as well as being excellent by itself or poured over some fresh rice noodles.
Of course I have heard the instant versions are fairly close and quite good as well. But where is the fun in that?
Place seaweed in large stock pot and cover with boiling water. Let steep for at least an hour then over a very low heat bring the water to a simmer, the longer this takes the better. Once simmering remove seaweed and increase heat. Bring stock to a boil then add bonito flakes and remove from heat. Let stand two minutes then strain stock. Dashi is ready for use or can be frozen. A second round of dashi can be made by simmering the used seaweed and fish flakes in four cups cold water for about 20 minutes then straining again. Discard the seaweed and bonito.