Pop-ups or a next button that leads to more offers AFTER completion of sale on the initial product/service purchased. Doing so before the completion of sale only irritates people and gives them a reason not to complete the initial sale. If you've ever bought a godaddy domain you've undoubtedly experienced this before. The copy is usually written to spark the ubiquitous "sense of urgency" you hear from everyone, with promises of discounts available on additional products/services only available then.
check out quickpaypro ... they seem to have an upsell option built in their payment software .... you want to do it after they've entered the CC info, ideally adding the upsell with just one click
Usually the upsell is something the merchant does (as opposed to the affiliate.) If you take a look at almost any low-priced ebook sales letter and then click on the "buy now" button, instead of going to a payment page, you'll go to an upsell page. Something like "wait, instead of getting this ebook for $7, get a special deal on this entire package for $97 instead. or pass this offer up forever and continue..." As an affiliate, you can't do much once you refer someone through your link. The closest thing to an upsell is when you offer someone a bonus gift for buying through your affiliate link.
Up selling and cross-selling are usually done before the transaction is consumated in order to increase the ticket size and there fore the revenue and/or commission. Upselling can really happen anywhere along the process or after the process. It can be done to previous customers and/or members that have joined your site or signed up for your newsletter.