Hanging curtains will help. Decrease the volume of space you are trying to heat. If the ceiling is unfinished, you will need to block the space between the floor joists or the warm air will flow out of your space.
Concrete will absorb a huge amount of heat. That floor mat is a must.
Eliminate drafts. It's hard enough to get the place warm. You don't need fresh cold air ruining all your hard work.
Heat the body rather than the room. Paul Wheaton has
an article on this. If you gotta use electric heat, this would be a highly efficient means.
My job has me working outside in all conditions, day and night. When it gets down to 18 degrees with the wind howling, no shelter in sight and another 6 hours left to the shift, guys will do just about anything to stay warm.
A knit hat is the first and best means of keeping warm. Next is layers. T-shirt + thermal wear + long sleeve shirt + sweat shirt(s) + hooded coat.
Sometimes we have access to a microwave oven. We always have bottled water. Pour out a splash of water, squeeze the air out of the bottle and recap it, then stick the bottle in the microwave for 3 minutes. Makes a fine cup of instant coffee, but stick a couple of them inside the pocket of a bottom layer sweatshirt, it will help keep you warm for a couple of hours. Its a hot water bottle.