This is just 2 things I've done which have made a big difference to my FPS.

1) Disable or reduce the size of you're windows paging file.
The windows paging file uses you're hard disk as a kinda of virtual memory for reading and writing data during game play. Wow is very hard disk intensive and having to use the same hard disk to read and write virtual memory + load in game objects and items can cause you're FPS to suffer.
By simple removing OR limiting you're paging file in windows to Lower(256) Upper(256) you "MAY" notice a FPS increase depending on you're hardware.
"PLEASE NOTE: I don't advise you try this on systems with less then 2GB memory for Windows XP or 3GB memory for vista"


2) Have windows media player open while you are playing Wow.
Now you're properly going to say "hold on a minute, its not Aprils fools day yet..." well this is actually a documented fact that windows media player can help to increase you're FPS by up to double what you're currently getting.
Again this is all hardware dependant and I do not guarantee an FPS increase.

Here's a link to more information as to "How" this helps: Wikipedia

and a snippet of the brief overview:

“Multimedia Class Scheduler Service (MMCSS) is a Windows service that boosts the CPU as well as I/O priority of a thread. It allows an application to get prioritized access to CPU for time-sensitive processing (such as multimedia applications) as well as prioritized disc access to ensure that the process is not starved of data to process. The MMCSS service monitors the CPU load and dynamically adjusts priority so that the application can use as much CPU time as possible without denying CPU to lower priority applications.”

Any questions or comments please let me know