I believe Clickbank also have a one-click upsell feature now but I haven't looked into it yet...

The regular way is also simple to set up, but I respectfully disagree with Drew above as you may lose a huge number of people before the Clickbank Order Form.

The OTO process that has worked well in the past has been:

Salesletter ---> Clickbank Order Form ---> Customer purchases front-end ---> OTO ---> If yes, another Clickbank Order form / If no, send them to customer opt-in & members area.

Obviously you can introduce downsells etc in there, but I couldn't draw an upsell tree here