Seed Starting Soil Mix.

In the spring, at the end of the day, you should smell like dirt.
~ Margaret Atwood

To create a good base to plant your vegetable and herb seeds, mix together:

2 parts aged compost
2 parts coir (or if not available, sphagnum peat moss)
1 part vermiculite, perlite, or sharp sand

Compost
Compost is what's left after the aerobic decomposition of organic matter (such as manure, autumn leaves, plants...) When added to soil, it improves the soil structure, nutrient content, and moisture retention properties.

Coir
The outer fibers of a coconut shell are added to soil for their moisture retention properties. It is an eco-friendly alternative to sphagnum peat moss.

Sphagnum Peat Moss
A decaying, compacted moss grown in bogs and mires in the Northern Hemisphere, is often added to soil mixes because of its moisture and nutrient retention properties. However, the use of peat moss is not a sustainable practice because the harvesting of it is threatening the existence of the thousands-year old bogs. It should be used sparingly, if at all.

Vermiculite and Perlite
Minerals that when added to soil aid with moisture and nutrient retention, increase air circulation, and improve drainage, all of which are optimal for proper root growth.

Sharp Sand
A jagged, angular sand is added to loosen compact or clay heavy soils to improve air circulation and drainage. It is also used in seed starting mixes to encourage proper root growth.

For more information on potting soil mixes, Organicgardening.com offers these recipes.