I have been using coconut coir as a base http://www.wondersoil.net/store/wonde... (a bale of peat would probably be cheaper).
I add to that special kitty unscented cat litter from walmart ~3 bucks for a 25lb bag. The cat litter is to increase the mineral content and for moisture retention. The litter is just clay and searching on the internet for koi pond substrates you can find some analysis of the walmart brand special kitty unscented cat litter. Typically they have found it to be fairly high calcium and iron content which the coir tends to lack. http://www.cichlid-forum.com/articles...
I mix in about 1/6 to 1/4 litter by volume (once the coir is expanded). Add in some Epsom salt (magnesium to balance out the calcium in the litter. Using the application rate on the Epsom salt bag. And start my seeds in that.
I was a bit crazy and started my seeds 3-4 months prior to my last frost and also had to grab some liquid fertilizer at about 2.5 months since across the board they showed signs of phosphorus and nitrogen deficiencies. I have applied it twice at half strength.
While planting up my seedlings I have used nearly anything I could get my hands on once I ran out of coconut coir, like potting soil mixed with kitty litter, my garden soil, and coffee grounds (by as much as 1/5 of the total composition). Using my garden soil unsterilized may seen odd to some folks but the plants are going to go out there at some point so they better get used to the flora my soil contains. If a 4 inches plant cant take it I don't want it in my garden. I have had no issues, though I do find critters on top of the soil when I first flick the lights on.
So seed starting
Base coconut coir(fresh water washed) 4/5 or 5/6ths volume
Litter 1/5th to 1/6th volume.
dash of Epsom salt (for Mg)
any thing and everything.