The last Avangate newsletter have a very nice cross-selling tip: Cross Selling benefits
"Implementing Cross Selling schemas into your selling mechanism can have a major impact on maximizing your profits with a minimum effort!
The Cross Selling feature works as a suggestion box. Each time a customer adds a product into the shopping cart, the system displays a list of up to five complementary products that can be also added into the shopping cart without spending additional time on searching for these products.
For example, if the customer adds an Antivirus software solution into the shopping cart, the system can be configured, via Cross Selling feature, to automatically suggest complementary products such as extended Virus Definition Databases, Firewall solutions or Spyware Protection Software.
This is a great way to improve both customers' shopping experience and profits without using an aggressive selling method or appear intrusive to customers.
The Cross Selling feature is accessible from the "Marketing Tools" tab within the Avangate Control Panel. Please read the explanation before starting configuring the Cross Selling experience!"
I think that Cross selling is a great tools and it must be used by all software vendors for their ordering process. Of course, there are a set of rules that must be applied: be relevant, be natural (don't add a lot of buy options because you will risk to lose the customer focus) and set fair discounts.