Increase the number of wire turns in the coil
Increase the current through the coil
Use a softer iron core
Use a larger core with better magnetic properties
Reduce the air gap in the magnetic circuit
Wind the coil more tightly and evenly
Use thicker wire to allow higher current
Improve the power supply to deliver more current
Reduce resistance in the circuit
Keep the coil from overheating
Add more layers of winding if space allows
Use a closed magnetic path instead of an open one
Increase the coil length only if it allows more turns and current without overheating