This instructable is an extension to the Datalogger for Arduino Yun. This lets you use the Yun to log your sensors to the SD card, and uses Python and the Dropbox API to store the logged data periodically so you don't lose your data to the elements.
The link to the Github repository is here : https://github.com/ankitdaf/yun-datalogger-dropbox
Create a Dropbox account if you don't have one already. You can do that here : http://www.dropbox.com
Next, go to the Dropbox Apps Console here : https://www.dropbox.com/developers/apps/create
Create an app with the options shown in the image.
Note down the app key and the app secret that you get when you create this, we will use it later.