This shouldn't come as news to anyone, but the older your iPhone or iPod gets, the worse its battery performs. Don't blame Apple: it's just the nature of rechargeable batteries.
OK, you can blame Apple for one thing: You can't just swap in a new battery--not without some warranty-voiding surgery. However, you can plug in external power, which is cheap, easy, and affordable.
For example, HandHeldItems.com has a pair of 1,000mAh iPhone backup batteries for $13.99 with free shipping. That's after applying coupon code 2x1000mah at checkout.
Since you will usually pay at least that much for a single external battery, it's a pretty spectacular deal. And these will work with iPods as well.
But why do you need two of them? Simple: one for your briefcase and another for your car. Or one for you and one for your partner. Or just to have a secondary backup for when you're taking a long trip.
There are a couple things I particularly like about these power supplies. First, they charge using the same connector as your iPhone, so you can plug them into any sync cable or dock.
Second, they have big LEDs that indicate their own charge status.
What's the downside? Because these are 1,000mAh cells, they can't fully recharge a drained iPhone, which has a 1,150mAh battery. However, you should be able to extend your run time considerably.
I'm neither certain as to how long this coupon code will remain active, nor if these batteries will sell out. But I do think that if you want the convenience of having backup power available in two places, this is a hard deal to pass up.