Sale_Price Attribute
THIS IS UNITED STATES ONLY (for the moment)

For this to work, you must have the original price and the discount/sale price visible on the product page. The start and end date im sure are self explanatory but if you want to start a sale in the future you can type in the sale price to the "Sale Price" field, and enter the start date in the component. This means it knows the discount is about to happen but wont show it in Google until the correct date happens.

For everybody else outside of the US, leave this turned off in the global settings, as Google will fail your feed otherwise.

Googles links
Questions and answers about Sale Price and its sub attributes
http://www.google.com/support/forum/p/base/thread?tid=4d3a05888e738604&hl=en

Detailed information about sale_price attributes
http://www.google.com/support/merchants/bin/answer.py?answer=1196048&topic=30064