You can be bull rushed, grappled (with the movement option on a later grapple), or repositioned out of a saddle.
Unseat lets you get a free bull rush on a successful attack while dealing damage. It also means you are using a weapon to perform that attack and can gain its bonuses on the CMB check.
Stay in saddle is for a bolting mount or taking damage while mounted not to resist effects intended specifically to remove you.
Mounted combat provides huge benefits to face down unprepared foes but someone trained with appropriate techniques can counter with decent planning.
Buckling up is not really an option given by the game. Being tied to a horse well enough to prevent any possibility of falling could easily lead to ugly consequences like being dragged or considered helpless according to the rules.
As a DM I personally would allow you to use a military saddle to gain a +2 to CMD to avoid being dismounted and would work with you to create a feat to allow you to use your ride skill to modify CMD against attacks that remove you from the saddle if I thought a player was really being hurt by this.
Fair enough. Like I said the rules don't cover this issue in much detail. On a flying mount it shouldn't be difficult to tie yourself in in such a way as to allow you to fall from the saddle without falling completely free of the mount. If I were dealing with a setting like Dragonlance there would almost certainly be feats and/or equipment designed ing specifically with this issue in mind. And with a PC a DM should work with them on matters like this. All of the listed options require exceptional mobility to do to a flying player unless he has already hit near ground level. If you are really worried though look into trying to get access to feather fall.
Readied characters should give off some sort of visual clue that they are prepared. You may be forced into switching to a ranged attack but that still a viable option in many cases.
*edit shouldn't, not should be difficult*