There are a lot of good points brought up on EpicAdvice.com - Tools for lowering latency lowerping and smoothping both seem to get decent reviews, as well as a blog on elitistjerks that talks about some registry hacks, addon issues, and many other tips for reducing disconnects (which may also help with pings).
gaming.stackexchange.com/users/191/gnarf
I personally use Leatrix Latency Fix .
It dropped my latency from ~250ms to ~60ms on all European realms I tried (currently on Draenor).
gaming.stackexchange.com/users/1348/Ondrej Slinták
1) Are you on wireless?
If yes, plug it in and get an instant boost.
2)
Do you have other things using your connection?
My wife and I both play (different computers) and we're at 200 ping.
She turns on internet radio, bam we're up to 400 ping.
Run a netstat to check for existing connections.
gaming.stackexchange.com/users/1413/corsiKa
Well latency is primarly based on your distance to the WoW server and the network connection you have.
Are you in US playing on EU servers or vice-versa?
Is your Internet connection shared?
Are you uploading / downloading or your neighbours sharing your connection while you play?
All this can affect latency.
gaming.stackexchange.com/users/193/Trefex
You can use a SSH tunneling service, such as http://lowerping.com , these can help if you are physically far away from the WoW server.
They have a free trial to see if it helps in your situation.
Addons are unlikely to affect network latency that much.
gaming.stackexchange.com/users/195/Niall
This thread explains how and why the proxy trick works.
http://forums.worldofwarcraft.com/thread.html?topicId=6214123217&sid=1
"This is going to get technical in places; skip the bits you don't understand."
It follows that the non-proxy registry trick (see the answer about leatrix) is going to wreak havoc on non-gaming stuff, like downloads filling your upload capacity.
And I certainly concur about not using wireless
- I don't use anything wireless except for my laptops and mobile.
Thus I've never had mouse nor keyboard run out of batteries in the middle of a boss fight.
gaming.stackexchange.com/users/2535/MaHuJa
If you're playing over a wireless connection, make sure you have a strong signal to your wireless access point.
Also make sure your computer and access point are both configured to use the most recent wireless specification available (802.11n is newer than 802.11g is newer than 802.11b).
If you're playing over Ethernet, make sure your network adapter is configured to use full duplex on the highest possible speed (10/100/1000 Mbps).
Close any programs on your computer that might be using a lot of bandwidth.
Common culprits are streaming music and file-sharing (torrents, FTP, etc.).
gaming.stackexchange.com/users/329/Billy Jo
A change of DNS server is unlikely to make any difference to your latency, all DNS does is tell your computer the address of the WoW servers, it should only need to do this once per server (per session) at login or zone change, so you really won't notice DNS resolution speed differences.
gaming.stackexchange.com/users/263/GAThrawn
Sometimes it helps to turn error correction of Your internet connection off.
That is, if the high latency is caused by high traffic, rather than long distance to the server.
Look into the settings possibilities of Your router.
However, You don't have the guarantee that the provider is going to respect the router's query to turn error correction off.
Some providers offer the possibility to do this explicitly, either for free or a small fee.
gaming.stackexchange.com/users/103/Dave
Recently I've went from Brazil to New Zealand and tried playing on same realm...
I was amazed at first, my ping in NZ went up from 200 to 1k, but it was understandable due to distance...
But I was even more stunned when it actually dropped, in less than 1 hour, back to 200.
When I came back to Brazil, the ping took just few minutes to adjust.
Now, that proves to me WoW itself already do a lot of ping optimization.
So, I wouldn't bother trying too much other things as you can most likely disrupt its own system than help your ping.
The best thing to do is definitely tweaking your own machine for optimizations.
Things like what people already suggested here, such as the Leatrix (thought it really didn't work in my case, on wifi) and getting a cable if you are on wifi.
gaming.stackexchange.com/users/2349/cregox
Also give a try to wtfast.com , i personally use it.
With wtfast i get ~100ms while without it i'm over 250ms.
gaming.stackexchange.com/users/None/
I can often have a latency of over 600ms when raiding and I have heard people in-game say that latency can be reduced by using a custom proxy or DNS server (I don't remember which).  Are there any good strategies to reduce latency in WoW, in addition to loading the minimum of addons?
How can I reduce latency while playing World of Warcraft?
gaming.stackexchange.com
<world-of-warcraft><performance>
gaming.stackexchange.com/questions/676
gaming.stackexchange.com/users/212/Antony
You can use a SSH tunneling service like Leatrix Latency Fix. Make sure you have a strong wireless connection and close any programs that might be using bandwidth. You can use a SSH tunneling service, such as  http://lowerping.com , whih can help if you are physically far away from the WoW server. You can also use  Leatrix Latency Fix or wtfast.com. If you're playing over a wireless connection, make sure you have a strong signal to your wireless access point as latency is primarly based on your distance to the WoW server and the network connection you have. Check whether your neighbours are sharing your connection while you play. Also make sure your computer and access point are both configured to use the most recent wireless specification available. If you're playing over Ethernet, make sure your network adapter is configured to use full duplex on the highest possible speed and cose any programs on your computer that might be using a lot of bandwidth.
You can use a SSH tunneling service, such as  http://lowerping.com , whih can help if you are physically far away from the WoW server. You can also use  Leatrix Latency Fix or wtfast.com. If you're playing over a wireless connection, make sure you have a strong signal to your wireless access point as latency is primarly based on your distance to the WoW server and the network connection you have. Check whether your neighbours are sharing your connection while you play. Also make sure your computer and access point are both configured to use the most recent wireless specification available. If you're playing over Ethernet, make sure your network adapter is configured to use full duplex on the highest possible speed and cose any programs on your computer that might be using a lot of bandwidth.  Sometimes it helps to turn error correction of Your internet connection off.