Compare prices before you click buy
The simplest saving is to actually check whether the price is good. Many online shops change prices often, and the same product can cost different amounts from week to week, or vary between shops. A couple of minutes of price comparison before you add to the basket costs little and can save you more than any coupon code does.
That applies especially to larger purchases: electronics, furniture, travel. The more expensive the product, the more a few per cent of difference is worth.
Wait for the right moment
Impatience is expensive. Many products fall in price around the well-known sale periods, or when new models launch and the old ones have to go. If the purchase is not urgent, putting the product on a wishlist and waiting a few weeks can mean a very different price from buying straight away.
Price alerts and similar tools can make that waiting easier, since you avoid checking manually all the time.
Avoid impulse buys, the biggest leak of all
The single factor that probably costs you most over time is not the price of each product, but how many products you buy that you did not really need. A couple of simple steps help: put the item in the basket and wait a day before completing the purchase, or ask yourself whether you would have bought it without a campaign pushing you towards it.
This is not an argument against treating yourself; it is an argument that deliberate purchases are usually cheaper purchases.
Add cashback where you are shopping anyway
Once you have decided what to buy and when, cashback is a simple addition that does not ask you to change anything about the purchase itself. You click through to the shop via a service like Sparebørsen rather than going there directly, shop entirely as normal, and a share of what you spend comes back to your bank account afterwards, typically within a few weeks to a couple of months, depending on the shop's deadlines.
It is not a replacement for the other steps, but a layer on top of them. Price comparison finds the best price, patience and deliberate choices cut out unnecessary purchases, and cashback gives a little back on what you were buying anyway.