If anyone's ever read mel bartholomew's Square Foot Gardening he recommends mixing your own soil for raised beds.

Potting soil is pretty simple. It's not soil so much as a medium to allow proper moisture retention, promote root growth, and a little bit of nutrition.

The basic ingredients are : compost, peat moss, and vermiculite. There are other ingredients that can be added depending on the needs of your plant. Here's some recipes:

All-Purpose potting mix

* 8 quarts potting soil with vermiculite or perlite
* 1 quart coarse sand
* 4 quarts sphagnum peat moss, compost, and/or rotted manure

Lightweight, Enriched Potting Mix

* 8 quarts potting soil
* 1 quart perlite
* 1 quart vermiculite
* 8 quarts sphagnum peat moss
* 1 cup greensand
* 1 cup gypsum

Acid-Loving Plants Recipe (Azaleas, Camellias, etc.)
2 parts potting soil
2 parts coarse sand
2 parts peat moss
1 part leaf mold
1/3 part well composted manure

Cacti and Succulents Recipe
1 part compost
3 parts pumice or grit
1 part coarse builders sad
1 part of fine silt


Alpine Plants Recipe
2 parts coarse sand
1 part peat moss
1 part pumice

Orchids and Bromeliads Recipe
6 parts Osmunda fiber
1 part of 1/2 -inch charcoal

Sources:
http://www.bhg.com/gardening/container/basics/make-your-own-potting-mixes/
http://www.the-artistic-garden.com/potting-soil-recipes.html
http://www.backyardgardener.com/soil.html