If you have an ecommerce website then you should try and ensure your site is set up in such a way that you can easy advertise additional products. If for example you are selling electronic equipment then you should have an area come up that shows other products that they may be interested in. This may be batteries or cables etc. You may be surprised by how many additional products you cans ell in this way. Even if a customer knows that they can get the add-on item slightly cheaper elsewhere, they may just buy it from you because you have made it more convenient to do so.
Another way to upsell is by offering free delivery when you spend over a certain amount. You may want to add banners to your site that state free delivery when you spend x amount but also adding a message to the checkout page telling customers if they spend x amount more they qualify for free delivery. All these subtle little messages and ways of displaying additional products can make a huge difference in your overall sales and profitability.
Some sites show an area which displays other products people bought when purchasing that one. These products may or may not be linked to the purchase, but this is just another way of showcasing products.