I'd suggest the 3G Juice. It's an external type of battery that you just plug into the 30-pin port on the bottom of the phone. It's sufficiently powerful; if I plug it in when my battery is between 10% and 20%, it charges it all the way back up to full and still has a tiny bit left. Perhaps best of all, it's a Lithium Polymer battery (rather than Lithium Ion), so it doesn't naturally "leak" charge when it sits unused for any period of time. Supposedly you can charge one, throw it in your glove box for a full year, then plug it in and find it still has a full charge. I've never left one that long, but I can tell you I've never seen any discharge after sitting at room temperature for 4-6 weeks at a time.
As a side note, when I first got an external battery, I got the Kensington one. I was very disappointed with the amount of charge it would hold and provide to my phone, so I returned it. I read about the 3G Juice and decided to give it a shot. I love mine.
P.S. -- I also love that I can plug it into my phone when I need it and keep my phone slim the rest of the time rather than make it bulkier all the time as many of the battery/case combo things. IF that sort of thing is important to you, it's another thing to consider.
