Well, I for one hate things like getting Dismounted in the Booty Bay Tunnel, or on the Hellfire Ramparts, or in Black-rock Mountain... After some research, I've finally figured out how to get the MOST distance on a mount. Probably the best spot, however, is Warsong Gulch Tunnel - using this method, I can get to the main flag room while mounted in the tunnel about 60-70% of the time.
Step 1) Mount up!
Step 2) Get a fairly good distance between you and the "dismount line" (sometimes you need to guess what this line is).
Step 3) Start running STRAIGHT towards where you want to run through. It's critical that you run STRAIGHT. You'll want to use num-lock if possible, but if not, just hold the forward arrow. Note: For many people, holding down the right-mouse-click will guarantee that you don't run straight... So don't hold it down.
Step 4) Jump so that you will land about 4-5 yards before the dismount line.
Step 5) Keep moving STRAIGHT. It's important to note that any deviation in your movement speed or distance will cause you to dismount sooner.
The reason this works is because of the server-update-position function. Many people believe that you simply get farther due to latency, but this is NOT true. The problem is that the server can't be constantly updating and checking your x-y-z coordinates, so it only does it as often as it seems necessary. Jumping and strafing are ways that force the server to update your position. By jumping right before the tunnel and moving straight, you force the server to update your position and thus it won't update it for another 5-10 seconds.
I've found through my own trials that I'd stay mounted 3-4 seconds longer using this trick. Hope it helps!