If you don't mind performing some surgery that makes your 3DS a bit fatter, Japan Trust Technology has a method for extending your 3DS battery life.
JTT's expanded battery device, released today, gives your 3DS 5,800mAh, 4.4 times the system's standard battery. Your 3DS will become 9mm thicker, but JTT says you'll get over 10 hours play time.
The battery is available in two sets: with white, red and pink covers for ¥9,980, and with blue and black covers for ¥9,480.
Access the product information page for ordering instructions and pics showing how to swap out the battery.