The best recipe I've seen for potting soil can be found here.

It's from tapla (Al) on the Container Gardening Forum, and is a fascinating discussion of the needs of container plants.

Potting mix generally is peat moss and vermiculite or perlite so you would be mixing peat moss and vermiculte with more peat moss and perlite. If you add some cow manure you will be adding some nutrients but not many. Tapla has some good mixes but if you can find compost and some other organic matter to replace the peat moss, some renewable resource, your mix would be environmmentally better.

Look up how they build their mixes for Square Foot Gardening, I haven't tried it myself, but the logic seems sound.

In general they use vermiculite/perlite, peat moss, and multiple (five or so) different "brands" of storebought compost.

The multiple types of compost is to cover the likelihood that a given bag of compost is only made from a limited number of ingredients. Since compost is only as good as its ingredients, better to have large sample size to cover the bases.