1) Alchemical ways
2) Magical ways
3) Mechanical ways
All the ways need to be reversable, like (get out if the mount dies). Really I'm looking for something like a "locked gauntlet-like"...(seatbelts???) or just a magical saddle?
Was thinking, if money wasn't an object, to create a saddle that uses the "Arcane Lock" spell as a base.
A frem ranks in the riding skill?
First there is the military saddle which gived a bonus to ride checks made to stay in the saddle.
Then you could get a wondrous item that gives a bonus to the ride skill.
If you want something slotless spurs, a saddle or a riding crop/quirt* could work. Else boots or a ring.
*got both words from an online dictionary as I'm no native speaker. So please don't mind if they don't fit.
This saddle braces the rider, providing a +2 circumstance bonus on Ride checks related to staying in the saddle. If you're knocked unconscious while in a military saddle, you have a 75% chance to stay in the saddle.
So what exactly is it that you are doing? A DC 5 Ride check to stay in the saddle is an automatic success with a few ranks in Ride (since skills don't fail on a nat 1). The only other cases I could see are some sort of jousting tournament with higher DCs or plans to give your mount Spider Climb and ride it up walls.
This feat is also an option, even if the prereq of 9 ranks in Ride makes most of it semi-useless.