The best way to obtain an Arby's roast burger coupon is to join the company's online coupon email list. This allows you to receive coupons for the roast burger, as well as other products, through your email account.
Arby's sends out various offers through the email list that can be redeemed at participating locations. Otherwise, the best way to get coupons for Arby's is by clipping them out of papers and menus that the company sends out in the mail, to residents near their locations. All you need to do is clip the coupon, and present it to the cashier when you order your meal.