Even though my iPhone 4's battery life is much better than my old iPhone 3Gs's battery life, I still need extra juice. I notice it most when I'm traveling, such as when I attend a trade show or similar event.
In these situations, I am often on-the-go from early in the morning until well past midnight and tend to use my phone a lot more for social media, map searches, email, etc. That means I often run my phone down by early evening. Not good.
The solution? For the past couple of months, I've been using a Mophie Juice Pack Air battery pack / sleeve for my iPhone 4 and I love it (I have the black version - the red ones weren't available when I bought mine - bummer).
The Mophie Juice Pack air is a thin, light sleeve for the iPhone 4 that not only protects the phone, it has a built-in battery you can use to fully recharge your phone.
The Mophie has a switch on it that lets you decide when you want to tap into the reserve battery. I tend to wait until I'm at about 15-20% remaining battery, then flip the Mophie's switch and let it recharge my phone. It has saved me on a number of occasions, I assure you.
The Mophie Juice Pack air includes the battery pack / sleeve itself, along with a cable you can use to charge and sync your phone to your computer (the only thing I don't like: the Mophie uses a Micro-USB port instead of a larger, more robust port (I think all Micro-USB's are a bit too flimsy for my taste).
So, if you use your phone a lot and find yourself running out of juice during the day, you'll love the Mophie JuicePack Air (yes, it's still small enough to fit in your pocket with the Mophie on the phone).
By the way - if you have an iPhone 3Gs, you can still get a Mophie Juice Pack Air for the iPhone 3Gs - my son is using my old phone and old Mophie, and it would be very difficult to get him to part with the JuicePack Air.