I'm going to assume for the moment that you're really asking "how do I cross-sell and upsell to people without compromising user experience", since that's actually a very interesting question.
Cross-selling and upselling are activities which frequently are focused on the using the customer as a resource to be mined for as much money as possible, and are therefore usually associated with aggressive sales behaviour. You don't need to do more than a simple Google search to discover that people frequently find these things annoying and irritating.
In addition to being annoying, aggressive sales techniques can have the effect of alienating users or driving them away. In the short term, getting someone to buy something they don't actually want may help your bottom line, but it's going to provide a poor user experience and that may not be a positive thing in the long run.
In order to maintain a positive user experience, cross-selling and upselling need to be approached from a user-centric perspective, rather than as a company-centric technique to boost sales. There is occasionally genuine value in cross-selling and upselling - some users actually do want to know that there are other products they could buy with the thing they just bought, or that there's a better version available. It's this value that you need to offer.
The heart of the matter is choice. Customers want to feel in control, and consequently need to be able to choose to upgrade their purchase or add to it. Typically, aggressive salesmanship takes away this feeling of control in order to push the sale - again, it's the attitude shift that's important - users feel like upselling is for the company's benefit, not theirs. In this light, the positive approach is to offer other products or upgrades, and tout the benefits - but without being pushy. Frame the offer as a suggestion that you're making in their best interest - not in a patronising way, but in an informative and helpful way.
The bottom line is: cross-selling and upselling needs to add genuine value to the purchasing journey from the customer's perspective to provide a good user experience.