If your car has been getting hard to start, the problem might not be the battery itself, but corrosion on the battery terminals. Knowing how to clean battery terminals could save you the cost of a new battery. The corrosion can be quite acidic so it's a good idea to wear gloves and eye protection.
The first step when cleaning battery terminals is to remove the cables from the battery. Connect a battery saver if you have one to save your radio presets and other settings following the instructions provided with the device. If you do not have a memory saver make sure that you have any PIN numbers required for the radio or other equipment in your car.
Starting with the negative cable (usually black and marked with a '-') remove the cable by loosening the nut or bolt and then twisting the terminal back and forth while lifting it off the battery post. Tuck the terminal out of the way where it won't touch either battery post, the body of the car, or the other terminal. Repeat the process for the positive cable (usually red and marked with a '+').
The best way to remove corrosion from the battery is to use an old toothbrush and a few tablespoons of baking soda in a small paper cup filled with water. The baking soda will help neutralize any battery acid and will provide some grit to help when cleaning. Apply some of this solution to the posts and terminals and allow them to soak. After waiting a few minutes use the toothbrush to scrub them until the corrosion is gone. Use a rag to remove the water and baking soda solution and dry the battery and terminals. If this doesn't get rid of all the corrosion you may have to use a wire brush or piece of fine sandpaper on the stubborn areas, however use these sparingly as its possible to remove so much metal that the terminal will no longer fit snugly over the battery post.
Reconnect the battery cables starting with the positive cable, followed by the negative cable. Be sure to verify that you connect them properly as reversing these cables can cause damage to your car. Remember to remove the memory saver device if you used one. To prevent corrosion in the future use battery terminal protection spray or cover the terminals with dielectric grease after reconnecting.