There are two very good solutions I've used.

The XPal XP2000 (http://www.amazon.com/XPAL-XP2000-External-battery-adapter/dp/B002V0NO96/ref=sr_1_2) is small, very reliable, and has 2000mAh of capacity. The company has since been bought by Energizer; the link above is for the XPal version at more than 50% off the same unit from the new company. It's a screaming deal, don't pass it up if it meets your requirements.

It has enough juice to recharge my iPhone from zero to hero, but it doesn't provide enough current to recharge my iPad. I relegated it to second-line service and purchased a New Trent iPulse, with 10,000 mAh capacity, two ports and enough current to recharge an iPad. It doesn't appear to be available any more, but I would recommend New Trent to you if the XP2000 doesn't have enough capacity for your purposes.

With any lithium ion rechargeable battery, you want to make sure that you charge it regularly whether or not you've used it. I recharge mine every month and after every time I use it.