Purchase Coolmax sheets, which cool the temperature of your bed. Sheets with Coolmax technology allow your body heat to pass through the fibers of the fabric, allowing for more breathability and less body heat retention, which makes your memory foam mattress cooler. This makes your memory foam mattress firmer, because memory foam mattresses become firmer when they're cooler.
Add a mattress topper or pad to your memory foam mattress. The added padding can provide you a firmer foundation upon which to lie.
Set your thermostat to approximately 65 degrees Fahrenheit. Again, pulling from the principle above, because memory foam mattresses adjust to temperature, a cooler temperature makes your mattress feel firmer.