In the eCommerce game, there are some tactics that were once considered speculative that are now commonplace and treated as if they were 
always commonsense. Upselling is definitely a good example of this. Once it was almost considered a specialized talent, then Amazon showed that you were giving away money if you didn’t do it. In short, upselling is getting a customer from one purchase to two, two to three, three to four, etc.
The basic principle of upselling is that you recommend the customer add another item to their cart. However, you need to be specific about which item they add and it needs to be relevant. Simply saying, “Would you like to buy something else?” isn’t effective. It also isn’t going to work if you say, “Would you like batteries with those shoes?”
Sometimes the pairing will seem like common sense. If a man purchases a shaving razor from you, it makes sense to suggest buying replacement blades. You probably don’t need a lot of research to understand that’s a pairing that works. However, it’s still worth checking your analytics to see what most customers who bought shavers added to their cart. What if they do add razors, but they also add lotion too? Would you have guessed that? Now you have an upselling opportunity when a customer buys shavers and replacement blades.
This tactic does have its limits though. Whether you know it or not, you’ve been upsold in real life or at least someone tried. Have you ever bought a car from a dealer, for example? Many people have to contend with a salesperson who tries upselling them on leather interiors, power or heated seats, sunroof, etc. Eventually, it’s all they can do to not just walk off the lot. Don’t be that salesperson. Make a suggestion and if they decline, let them continue to checkout.
However, you can do more than just suggest an item. You can also offer them a discount if they take it. Another option is to tell them that if they take the upsell, their total price will make them eligible for free shipping. Better yet, tell them the added cost of the upsell makes them eligible to save a certain amount on a third product.
So long as you’re respectful, honest and understanding if the customer turns you down, upselling is a tremendous tactic for making more off each sale. If everything else is the same between you and a competitor, the one who can upsell better is going to be the one who wins out.
Source:
http://www.flow20.com/5-upselling-techniques-to-increase-your-online-sales/