What, if anything, determines a survivor's success at fighting a Jockey's attempts to move them in a direction?
One thing I do as a survivor that seems to help is to repeatedly tap my directional keys in the opposite direction the Jockey is trying to move me. This appears to be more effective than just holding the key down continuously.
Here's some advice from the L4D wiki:
The Jockey controls relative direction of your movement (forward, left, right, backward), but not your facing, so a tactic to prevent being ridden too far away from your team is to spin repeatedly and quickly in circles. This generally makes it easier for a teammate to run up and melee the Jockey off of you, but be aware that it can also make it more difficult for them to shoot him off, as your movement will be highly erratic.
I tend to run up and melee my teammates with a Jockey on their head and then shoot it, as it's usually a quicker way to save their health bar than just shooting, especially since the update where they increased the Jockey's DPS in Versus.