Hi G,
I use a basic peat-based potting mix.
Two years ago (when I first started RGGS) I just used whatever commercially bagged potting mix (Lowe's Sta-green brand or Mircle Gro) I could get and some lime & espom salt AND whatever amendments (gyp etc) with the ubiquitous ring of 10-10-10 fertilizer on top.
Last year I mixed my own using Larry Hall's DIY potting mix recipe:
2 (5-gallon bucket) peat moss, dry
2 (5-gallon bucket) rotted manure/compost
1-1/2 cup garden lime
12 cups perlite
handful epsom salt
Watch his youtube movies (like his hybrid one where he shows mixing the mix and filing each net cup - tons of info in that one, and its long.)
Ricman on here has also had much success with it. Read message #14 - his mix is 60/40 of potting mix/compost. Its not what's recommended but he seems to have done great with it.
http://www.tomatoville.com/showthread.php?t=33037
If you want to trial the 3-2-1, please let me know how it works out for you. I'm just a little worried that there might not be enough peat to do the wicking from the bottom...