Explanation:
Increase the Voltage: Ohm's Law tells you that current is proportional to voltage, so if you're running your electromagnet on a 6-volt battery, switch to a 12-volt one. You can't keep increasing voltage indefinitely, however, because wire resistance increases with temperature until a limiting current is achieved. That brings you to the next option.
Lower the Wire Gauge: Wire resistance decreases with increasing cross-sectional area, so reduce the wire gauge. Keep in mind that reducing the gauge is synonymous with increasing the wire thickness. If you've wrapped your solenoid with 16-gauge wire, replace it with 14-gauge, and the magnet will be stronger.
Lower the Temperature: Resistance increases with temperature, so if you can maintain your magnet at below-freezing temperatures, it will be stronger than one at room temperature, although the difference probably won't be much. At extremely low temperatures, however, resistance almost disappears and the wires become super-conducting. This fact allows scientists to design uber-powerful magnets, such as the ones at CERN.
Use Wire with High Conductivity: You can also increase current by upgrading to a wire with a higher conductivity. Copper wire is probably the most conductive wire you can use, but silver wire is even more conductive. Switch to silver wire, if you can afford it, and you'll have a stronger magnet.
