I recently purchased a portable USB power supply. This one in fact.
According to the description, it holds a charge up to three months. It’s really small, which is great, it drops right into my bag without noticeable bulk or weight. I was out one evening and noticed my Blackberry battery was nearly depleted. I plugged it into my newly purchased and charged portable power supply. In about an hour, it was fully charged. All while I relaxed in the bar and didn’t miss a tweet.
Inexpensive, small and it does exactly what it says. You can’t ask for more!