Keep the pressure up

You're walking around your home turf, tearing rival gyms down and dropping a mon inside. Fifteen minutes later three mons sit inside the gym and it's nicely coloured the way you prefer. After your walk you've cleaned out five gyms and walk home satisfied with your progression.

It's just that it doesn't work. Another two hours later it's all gone.


Why?

A newly won gym effectively has three slots free. When those are filled i has 6K prestige and three pokemon inside. That's two round of battle to clean out. Maybe three minutes if the pokemon inside are grade A defenders. More often than not a little over a minute from my own experience, because people drop crap inside those gyms.


Pressure

Pressure starts at 20K prestige. That's seven pokemon inside. If the gym doesn't go down hard, other players are prone to keep it at 20K prestige, even if it gets kicked down a notch or two.


Deterrent

Starts at 30K prestige. A level eight gym is a big one. Determined players might go the extra mile and increase prestige to 40K, and from there to 50K.


Keeping the pressure up

Just tearing down and dropping something inside won't work in the long run. You need to create pressure.

Obvously you're unlikely to manage keeping all the gyms around you, but if you push a number of gyms to 10 whenever they go down you'll see a pattern where some of them are simply left as is. Focus on those. They are your territory.

Eventually any gym will go down. The best response is to immediately build it to level ten, or at least seven. If you do those gyms will become known among players in your team. The hard-core ones are likely to pay them an extra visit and slot inside something nasty, and you want nasty stuff to populate the gyms closest to your home turf.