The New 3DS will correct the lack of a second analogue stick when it releases this year, but if you've got a 3DS XL, the Circle Pad Pro peripheral is an ugly yet functional solution to the problem. It's a big plastic tray with another circle pad on the side, and though the games that support it is limited, the likes of Metal Gear Solid 3: Snake Eater 3D and Resident Evil Revelations are massively improved. £5.86 is an absolute steal, and the rounded base actually makes the console more comfortable if bulkier.
Remember that this will only fit XL models, not the original 3DS. Thanks to jony_bobo @ HUKD!