Yeah, like others said, you have to keep the toys in their boxes plus you have to take care of the boxes as well. Things as simple as a scratch on the cardboard or plastic covering... hell, even imperfections in the tape can quickly devalue collectors toys.
But even if you keep the box in completely perfect shape, it's still a shot in the dark as to whether it will ever be worth anything. There are so many deciding factors. The amount of mint items that exist, the interest in the item and the ease of getting them. Unfortunately, for collectors looking to have their possessions skyrocket in value, you have more and more people saving their toys, which of course makes them less rare. So while your toy might be valuable in 20 years, I would say the odds are more likely that it won't be... but you won't know for sure until 20 years from now.
On a separate note, can I just say how much saving toys in the packages just irritates me to no end? It's become such an obsession now. For my birthday, my brother bought me a Junkyard Dog action figure and gasped when I immediately opened it up and displayed it on a shelf. Honestly, the toys are meant to be enjoyed and I personally think the enjoyment you get is worth more than the very slight possibility that it might go up a few dollars decades down the line. (this isn't a criticism or you, Meme Boy, or anyone else, so please don't look at it that way... just my own thoughts about the practice).
