USE MORE COILS OF WIRE AROUND THE NAIL
Explanation:
To increase the strength of an electromagnet, the following strategy is adopted in order to have better efficiency.
The best method which suits for increasing the strength of electromagnet is to increase the number of turns around the nail.
If we increase the number of turns up to a specific value the magnetizing property of magnet is enhanced. So that to increase the strength , add more number of turns to the existing turns of coil.