I usually caught on to whether it was a mail-in thing or not before buying the cereal. It was still annoying that you had to send in for the good prizes though.
What we did when I was a kid was simply save all boxtops/UPCs/etc, even when there was no current promo. Then, when one did pop up, I'd already have 6 of the 8 required PoPs. (I think companies caught on to this tactic though, since they later started requiring PoPs with dates on them & the dates had to fall within a certain span.)
I don't recall any specific cereal mail-ins that I redeemed, but I did kool-aid stuff a few times (one specific one was the plastic pitcher & cups), a couple canned pasta ones (I remember a chef-boyardee matchbox semi, and IO thin a franco-american one too), and a legions of power mini-set through quik (back when it was still "nestle's quik").