Every failed slab had too much water in it
The mix
One cement to two sharp sand to four aggregate by volume for a general slab. Measured with a bucket rather than a shovel.
The water
Enough that it holds a shape when squeezed and no more. It should be difficult to work, which is the part people cannot resist fixing.
Curing
Keep it damp under plastic for at least three days. Concrete does not dry, it reacts, and letting it dry out early halves the strength.
