Very simple, very important precaution:
Don't sleep alone.
A barracks-style sleeping arrangement, with, say 30 people sleeping in one room is enough to discourage people from sneaking or teleporting in and starting to kill your cult, mercenaries, thieves or whatever with impunity.
But the High Priest should have the privelege of rank and sleep alone, right?
Nope. In a cult you want to keep a small coterie of confidants near you, an inner circle. There's his group right there.
This does assume you have somewhat large rooms and are used to privation, but in a world with monsters and magic, sleeping in a room with 29 other people isn't too much discomfort if it helps you stay alive.
Another very simple precaution: Take up residence in an old temple.
Over the thousands of years of history most D&D worlds have, there should be a large number of underground or fortress temples to various deities, passing from one deity's followers to another, to a secular group, and back again.
The reason?
The temple has been blessed with a Forbiddance spell, preventing all planar movement within a very large area...except of course, for the teleportation room, where the spellcasters familiarize themselves with the layout of the room so they can escape to the bosom of their bloodthirsty cult brethren.
Sure, new temples will be built, and not *every* temple would have a Forbiddance spell, but if it's still there after hundreds or thousands of years, it's there for a reason.
Temples and other important buildings should also be built around geological anomalies, like hot springs, volcanoes, paint pots, geysers, and possibly large waterfalls. Alternatively, Permanenced spells that cover large areas are also an option.
Control Weather, Permanenced (or simply cast Extended every two or three days) is also an option, but a constant storm of some kind isn't always desirable.
These factors increase the failure rate of teleportation. Combined with the difficulty of knowing both the location and the layout of an area simply through scrying, this should discourage any but the most persistent and bull-headed of enemies.