Your alternator doesn't charge your battery the way you think it does.

It's a common misconception among drivers that if their battery is flat (by which I mean dead), a good long drive will charge it up. After all the alternator generates power, right? Well yes, and no. Have you ever seen or driven a car where, as you sped up, the lights got brighter? That's because the battery was dead and all the electrical power was coming from the alternator with no battery to act as a load-balancer.
Bear in mind here the distinction between a low charge, and a flat, dead battery. Yes - alternators can revive batteries that have drained somewhat, but they can't revive a dead one (or at least it's not recommended to try). When people say 'flat battery' they normally mean 'low charge'. A truly flat battery is one that is nearly dead.
In a properly functioning car, the battery is there to start the car and be an electrical load-balancer. After the initial current draw of the starter motor, the alternator takes over and runs all the loads (lights, wipers, radios and other electrical gear) whilst providing a topping-off charge to the battery. Run that battery down far enough (under about 10v) and either the alternator won't recharge it, or it will damage the cells and give them a surface charge that will draw off so fast that it makes the charge useless. The alternator can't really be relied on to recharge a fully dead battery. So much so that most alternators have a warning label like this on the box when you buy them:

The alternator in most cars is typically sized to match the demands of the running car. In other words, when the engine is on and the car is running, the alternator provides the required electrical load for all the equipment and typically doesn't offer much more than about a 5%-10% overhead for the topping-off charge. To fully charge a nearly dead battery could take a very long drive with everything electrical turned off and as stated above, could result in a near-useless surface charge (and/or a dead alternator). A quick 10 minutes down the motorway isn't doing anything for you.
That's why you should invest in a battery charger if you ever find yourself with a very low battery. Use the grid's power supply to charge your car's battery instead of trying to overload your alternator. Chargers come in many sizes from 0.75 to 1.5 amps for slow 'trickle' charging, all the way up to 200 amp quick chargers (although you'll need a dedicated electrical supply for something that beefy). Optimate, Battery Tender and Black & Decker all do reasonable chargers in the $20 to $60 range, available from most local car parts stores or online mega vendors, and using them is pretty easy.
They'll typically come with a variety of connectors - clamps, ring connectors and accessory plugs. The accessory plug might seem like the ideal choice but many modern cars don't have live 12v accessory sockets when the ignition is off, plus you're trying to charge a hefty car battery through tiny cables so I just wouldn't recommend that. Ring connectors are good if you need to charge the battery time and time again because you can wire one end of the charging cable permanently to the battery.
The most useful connectors are going to be the crocodile clips. Simple to use - connect the red one to the positive battery terminal and the black on to the somewhere on the chassis or engine, plug the charger in and leave it alone. A decent modern charger will be able to charge in three or four different modes - bulk charge, top-off and maintenance are the most common three. It should choose which mode to use based on the condition of the battery, and there'll be a light or indicator on the charger to tell you what's going on.
Two final points : some cars, notably high end Audis and VWs, have their 12v battery in the trunk or under the driver's seat. Typically they'll have charging posts under the hood somewhere for easy access - Audi put theirs under a plastic flap at the back of the engine bay, in the middle. Flip it open and there's a "+" post in there. For the negative terminal, just clamp the clip to any part of the chassis or engine where there's bare metal.
And secondly : if you have to keep charging the battery, it means either your alternator is about to give up, or more likely, the battery itself is about to give up. Expect 3 to 4 year's life out of a normal 12v car battery, irrespective of what the manufacturer or guarantee tells you. 5 years is out of the ordinary. 6 years is extreme. If you live anywhere with wild temperature swings, expect 2 to 3 years before you need a replacement.