There's a lot of theories out there for what's best for Bonsai, but after some years of trying various things I settled on a simple formula that seems to work for my standoff approach to the art, where I like to have the odd period where I shape trees etc, but mostly just treat them as any other potted plant. My wife is an expert gardener and she also uses this method. Any regular compost is fine, ericaceous for acers and others than need acid soils, and regular for anything else, avoid sandy types like John Innes no 2 or ones with loads of bark in them. Mix that with 1/3 coarse grit, and 1/8 garden loam (regular soil, but not chalky types). Ensure the pot has good drainage with gauze over the drain holes, and a layer of corse grit in the bottom of the pot. This formula can be watered till it's sodden, but will drain readily pulling desirable air down into the soil. The loam helps to keep moisture, but in essence its very difficult to over water. So, as long as you water regularly you don't have to worry about watering too much, it takes the sensitivity away from that side of the care.