Common to both conditionalists and eternalists[1] (those who think that the Wicked will be consciously tormented forever) is the assumption that one can get insight into the length and/or nature of the punishment due the wicked by contrasting it with what the Righteous will enjoy. One reasonably assumes that the wicked will receive the opposite of what the righteous receive, and notes what exactly the reward of the righteous is, and from that deduct the nature of the punished due the wicked.

For conditionalists this means that the wicked will eventually pass out of existence, since only the righteous will have everlasting life, even if their punishment is eternal. (However, as I see it, this argument rests on a misunderstanding of what "everlasting life" means.)

Keeping in mind that everlasting life is primarily described as union with God, where the righteous enjoy honor, joy and glory, eternalists claim that, since the conscious enjoyment of the righteous is perpetual, the wicked's torment will also be perpetual; and they're torment will consist in suffering shame, distress and humiliation at being perpetually separated from God, which is the opposite of the reward due the righteous.

I think this latter 'opposite argument' is stronger, since it seems to describe what Spiritual Death is, and I think that spiritual death, which is the wages sin pays (see Romans 6:23) implies perpetual, conscious existence. (More on this later.)

Additionally, it counters the notion that the 'eternal shame and contempt' due the wicked (see Daniel 12:2) isn't eternally experienced by the wicked. You might as well be suggesting that the honor due the righteous is primarily just God's viewing them a certain way. (Which is basically what annihilationists say about the shame due the wicked; they'll experience it for a while, but then will pass out of existence, however, their memory will always be held in contempt. More on Daniel 12:2 later.)

By these two examples, I'm sure you can see how such reasoning works, even if you can (and I eventually will) take things further by developing more complicated arguments. I think it is a plausible way of thinking, and, all things considered, favors the eternalist position. What do you think?

[1] I think this kind of argument is pretty damning (pun intended) for the universalist. It seems that they can't say that the wicked will not be punished eternally - because the Hebrew and Greek words we renders as 'eternal' or 'everlasting' don't necessarily mean what we mean by those English terms - without saying that the reward due the righteous isn't going to be eternal either.