Contact customer service - many times by chatting with a customer service representative they will give you extra discounts to complete your purchase.
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 multiple email addresses - sign up for stores email lists using a couple different email accounts. Stores will offer single use coupon codes, combine these single use coupon codes from multiple email accounts to maximize savings.
Set up discount email - to clean things up, set up an email specifically for collecting coupons and discounts without cluttering your personal email.
Free Shipping - always look for free shipping coupon codes or for the minimum free shipping amounts to score this discount.