Upselling is the art of selling the next consecutive product immediately after your customers have bought the first one. This helps you ride on his/her purchasing momentum.
The normal advice is that upselling occurs on the Thank You/product download page. Actually, if you can make a sharp distinction between the various Thank You pages you may already have on your site, you can advertise more products than you may think.
Here are some 'labels':
1) Newsletter Thank You Page
2) Paid Product Thank You Page
3) Members Area Sign-up Page
4) Use your 404 Code for your 404 error page at your site (sell a product every time visitors got your URLs wrong!)
5) Safelist Sign-up Thank You Page (if you run a safelist)
6) E-mail Confirmation Page
7) Opt-In Thanks You Page
8) Are there any more?
See? There are more places you can sell than you ever thought of. You can even include Adsense ads for additional income-by-clicks.
This is your homework: review all your Thank You pages and think of at least 2 products related to the subject matter of each page so that you can put them in.
Think of creative ways to make offers that are only exclusive to a few Thank You pages. Especially for the 404 error page, make your visitors feel they 'stumble' into a once-in-a-lifetime offer they can't resist, and you would hook them to a sale!