Raise the seat. If your seat is too low and your legs rise to an angle greater than 65-degrees in relation to your body, you exhaust yourself. If your seat is high enough so your legs extend fully when the crank arm is at the bottom of the cycle, the effort you expend is less. Of all the things you can do to make a single speed bicycle easier to pedal, this is the most important.
Fill up your tires. Bicycle tires are like car tires since, if they don't have the proper amount of air, they ride flat. Tires with low air pressure are harder to pedal because they have more surface area, creating additional friction with the ground. Fill your tires 5 pounds beyond the recommended amount to lessen the friction.
Change the gear ratio. If your single speed is difficult to pedal at slow speeds, purchase a front gear that is smaller or a back gear sprocket that is larger. If your bicycle is difficult to pedal at high speeds, increase the size of your front sprocket or replace your back sprocket with one that is smaller.
Adjust the size of your crank arms. Longer crank arms make it more difficult to pedal at low speeds, particularly when you first start, but are easier to pedal at high speeds because your legs have more leverage. Shorter crank arms make starting off easier, but, at high speeds, the revolutions per minute are so great that you will exhaust yourself trying to keep up.