Before you plunk down the money for a new battery, make sure you've covered all the other possible options. Start with looks. If the top of the battery has cracks in it, you need a replacement.
Next, check the battery terminals, which are the positive and negative knobs that the wires leading to the alternator are bolted onto. Older batteries can produce a sulfate build-up (a whitish-looking residue) that corrodes the terminals and leads to a bad connection. Sometimes a gentle tap with a hammer on the terminals will break away the residue and solve the problem. Or you can scrub them with a pasty mix of baking soda and water using a wire brush or an old toothbrush.
If everything looks fine but you've had to jump-start the car more than once, the problem may be that the car wasn't driven long enough to properly recharge the battery. About 30 minutes of constant driving should do it.
Note: When you drive, keep all unnecessary electric equipment off (such as the radio, the air conditioner or heater and, if possible, the headlights and windshield wipers).
Your car recharges the battery through the alternator, which supplies and distributes electricity after the car is started (when you start the car, you're using only the battery). The alternator gets its energy from the movement of the actual engine via the alternator belt. If your battery still isn't charged from extended driving, the alternator belt might be loose. Different car models require different belt tensions, so have it checked by a professional.
Another culprit could be the weather. Extreme cold or heat can affect battery performance--especially cold, which can freeze the electrolyte solution inside. If your battery has removable vent caps (located on the top), remove them and look inside to see if this is the problem (remember to always replace the caps tightly). If the solution is frozen, you'll have to wait for the weather to warm up, or tow the car into an enclosed area.
Finally, your battery might simply be old and ready to retire. If this is the case, go to your auto supply store and buy a quality model that's right for your car and climate (some types are better for hot climates; others are better for cold--consult the merchant).