The best brand on the market for external batteries are Hyper juice but they are expensive because they can charge your macbook iPad,iPhone,iPod Touch,Nano.Another great thing about hyper juice is that you can charge the battery 1000 times without no problems most battery only last 500 cycles and the hyper juice will last 1000 cycles but it start decreasing in capacity
http://www.hypershop.com/HyperJuice-External-Battery-for-MacBook-iPad-iPhone-USB-s/91.htm
if you have a mac too then hyper juice might the better option for you because you could use your battery for several things and just not your iPhone when you are out and about.
If you cannot justify the money to get a Hyperjuice then and you just want to charger your iPhone then you can use something called a Newtrent these are reasonable cheap and they work well i have one myself just when i need to tether with my iPhone save using my Hyperjuice battery i prefer just to use my hyper juice battery just to charge my iPad 9'7 & iPad mini.
Here is the link for the Newtrent Series
http://www.newtrent.com/store/iphone-external-battery.html
I must admit i do think the Hyperjuice is a better out of the two but they a both good battery but hyperjuice do come at a heavy price for there stuff but they are worth buying its got me out of sticky situation a few times
when i have been in need a plug to plug in my iPad or iPhone and my macBook
I have the 60Watt version of the hyperjuice it can provide 3 hours of battery to my macbook or it can provided two full charges to my iPad
A lot of Photographers buy Hyperjuice for when they are out in the field to charge there MacBooks and iPad
I hope my information has made you make a the perfect decision in your purchase and i hope that you are happy with it too.