In addition to cards from each set, I like to keep a copy of whatever promotion the card company was running with the set. In a recent repack I pulled an unopened pack of 1991 Topps. For whatever reason I didn't have any of the promo cards from this set. Problem solved!
If you pulled a promo card with a number starting with an X, you were an instant winner and by sending in the promo card, you got some random card. You were also entered into a drawing for the Grand Prize and Second Prize.
If you didn't get an Instant Win promo card, you could still send in the promo card, or a 3x5 index card.
The Grand Prize was pretty sweet, a complete set for each of the 40 years of Topps production. In 1991, Topps said that this prize had an approximate value of $125,000. With inflation and a probable increase in value, just the 1952 Topps set is worth $65,000 today (according to the last issue of Beckett I saw earlier this year).
The second place prize was a complete set of Topps from some random year. There would be 6 second place prizes. I imagine you were more likely to get a 1989 set of Topps rather than a 1952 set.
These types of contests were illegal in Canada, and there was usually some arithmetic quiz for Canadians to pass in order to enter. The tests were something like 3 + 5 = x. Oddly, the 1991 contest was only open to US residents, no Canadians need apply.
That tiny print was a lot easier to read with my 40-year-old eyes in 1991.