One of the most common jokes about McDonald's is their constant practice of asking "Do you want fries with that?". When you realise that fries (chips to some other parts of the world) have a 400% mark up, then you realise that to them it is not a joke.
Attribution: Flickr - Jason Tester Guerrilla Futures
This isn't just the practice of up selling, it is about opportunity selling and it is not just about tricking customers into something that they don't want or need.
Attribution: Flickr- One Way Stock
One simple example is an African restaurant that I know. The owner runs this restaurant and also imports and sells various African products and souvenirs. He was thinking of putting some of these as decoration in his restaurant. Yet here is a great opportunity, instead of just displaying he could sell them as well. Just like many chain cafes also sell their own blends of coffee.
Attribution: Flickr - One Way Stock
There are so many opportunities to sell when interacting with clients. A friend runs a massage business and while she is doing okay with many repeat customers, her profitability is not very high. So we are currently brainstorming products she can see along with massage therapy. We are thinking of things like herbal therapy packs to help with muscle injury or stress relief packs for people who come for massage to relax. None of them will be expensive but all of them will have a nice high markup.
One last item is don't put the up sell before the sell. I have actually had to walk away from businesses that refuse to sell me something without insisting I buy other products or services as well.
Remember it is simply about taking the opportunity when the customer is there to sell more products and services that they need or want.