So, while some people say the only way is to combine two low-pop realms into one, Drysc says that such mergers can only be a last resort:
There are a couple issues with merging realms though that we would need to take into heavy consideration if one were to be considered. As many of the lower population realms are PvP, faction balance has to be taken into careful consideration. Would a merge worsen, improve, or leave any faction balance unchanged? Would a merge move the population up too high, queuing or leading to queues for the realm after that point? Do we have realms of the same time zone and type that would merge well under these conditions? There also many logistical issues with merges which ultimately effect the end user, such as conflicting character and guild names. So, in the end, a merge would only be considered if all other possible options have been completely exhausted.So, what is there to do besides merge realms? Drysc calls that the "million dollar question," which I figure means that Blizzard doesn't know for sure, though at least a couple ideas come to my mind: They could have some kind of incentive to players who join low-population realms (that's what the Army is doing, I hear), or perhaps all but force new players to start out there unless they have a referral from another player. Of course any solution to this problem is going to create still more problems of another sort. What do you think would work best?
We're not just throwing recommended tags up on realms, and calling it a day. The recommended tag has done a lot of good for a lot of realms, but if we're seeing that they're no longer helping reach the type of population numbers we would like to see, then we would investigate alternate means. That's sort of where we're at now.