It seems that in certain regions of the US, Miracle Grow Garden Soil is quite good. However, in my region it is muck and is totally unsuitable.
The easist mix to obtain is a 5-1-1 mix of mulch, peat moss, and perlite or gravel. This works great but the mulch rots in 6mo or so and you must repot or else it becomes poor-draining muck. There are longer last mixes but they are harder to source materials for - coconut husk chips, turface, granite, hydroton...