Never pay full price online. Almost every major retailer has active promo codes floating around the internet. You just need to know where to look and how to apply them.
Leaving items in your cart for 24 hours is a classic hack. Retailers track abandoned carts and will often email you a 10% or 20% off coupon to incentivize you to finish checking out.
Additionally, always check social media. Brands constantly sponsor influencers with unique discount codes. A quick search of the brand name on Twitter or Instagram can instantly yield a valid promo code.