An easy way to apply discounts to sales is to create a coupon SKU. A coupon SKU is an item that, when added to a transaction, will automatically subtract its sell price from the sale.
Create a new SKU (see: Adding a New SKU), and name it "coupon" instead of a regular SKU number.
Whatever amount you enter into the Reg. Sell Price field will be the discount applied when the coupon is scanned. It is also recommended that you add a description, to save having to type one for every sale. Be sure to un-check the tax box and check the "QOH=0" box before saving.
Now if you enter the coupon SKU into the window during a transaction you will automatically apply the discount.
A quick way to use a coupon SKU is to print a label with a barcode for the coupon SKU, and stick to a card placed near your till, where it can easily be scanned whenever necessary. You could also create multiple coupon SKUs, with each having a different discount. (One called "COUPON10" with a 10 dollar discount, and one called "COUPON5" with a 5 dollar discount, for example.)