Answer
Transformer designers place the coils close together, and use an iron core. The core directs the magnetic field lines from one coil through the other.
Work Step by Step
For maximum efficiency, engineers want the maximum number of magnetic field lines produced in the primary coil to pass through the secondary coils. Stray magnetic field lines induce current in other metallic parts of the transformer that are not coupled to the output, resulting in (wasted) heat.