How to avoid concrete cancer?

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When I studied at TAFE, the teacher told us the steel reinforcement should not be contacted with soil and should be covered at least 40mm, therefore, the concrete can protect the steel reinforcement and avoid concrete cancer. But now I work on construction site, I saw many cases that the steel reinforcement just contact with soil when doing footings or ground floor slab. Because the ground is not very level, and the footing is not big enough (if you don't want to waste concrete), it's not practicable to avoid the contact between the reinforcement and soil. But this will cause the rust of steel and then cause concrete cancer. Are there any guy working on construction site? If any, could you please take some photos to show how to avoid steel reinforcement to contact with soil when doing footing or ground floor slab.