Keep patient shopping cart - by not completing your purchase quickly, merchants will reach out to you and offer better pricing or more coupons to encourage you to complete the purchase.
Use reward programs - check out such products as Swagbucks and Shopkick, these companies will give you special discounts and gifts for filling out surveys and shopping around.
Request price drop refund - some companies will give you a refund for the difference in price if the product you purchased drops in price.
Contact customer service - many times by chatting with a customer service representative they will give you extra discounts to complete your purchase.
Set up discount email - to clean things up, set up an email specifically for collecting coupons and discounts without cluttering your personal email.