Blizzard, this is a problem! A server should not have this terrible of a population. Here are my suggestions. Don't allow for free transfers for the horde, only for the Alliance. Offer an incentive for people to come to Kel'Thuzad horde. Give some bonus for a guild to come here. Give them free stuff. Anything that will encourage people to balance out the population. Allow for free faction changes from Alliance to Horde with some incentive for entire guilds to faction change. Maybe give them free gear. That's a great incentive. I'm sure people would disagree with this, but think about it, what's better? A guild having an advantage over all the others for one tier of content for about a week or two, or an entire faction on a server dying off? Another thing that would help encourage the Alliance to transfer off, evening out the the number of players per faction, would be to lower the population cap on the realm. Nothing runs people off of a server like a long queue time. This would annoy a lot of people for a little while, but I believe this could help in the long run.
Blizzard, I know you cannot force people to come to Kel'Thuzad Horde, but there's not much time left to save this server before no one would even think of coming here. No one wants to transfer to a dead faction. You cannot recruit for your guild, you cannot find people to quest with. You cannot do anything with a group because there's no one to group with.
Cross Realm Zones could have helped prevent people from leaving, but you have those turned off for Mists of Pandaria. Set up Cross Realm Zones with a relatively low population cap on Pandaria zones so that a more even number of Alliance and Horde can be in a zone for low population faction realms. The problem isn't that the Alliance are jerks and just camping places, though they are doing that anyway; the problem is that there are hundreds of Alliance in a zone and only 13 Horde. I am not exaggerating these numbers. If you go look at the Valley of the Four Winds' populations on the two different factions, you will see that the Horde has less than 20 people in the whole zone, while there's at least 20 Alliance at each quest hub.
I am sadly leaving the server with my guild since they don't want to deal with this and many other guilds are also doing the same thing from what I've heard. Pretty soon there will be fewer than 200 active members on the Horde, while there are thousands on the Alliance.
Blizzard, this is in your hands, there is nothing anyone but you can do to help this situation.