This works for my 4 o'clock right side carry.
Unbuckle belt and start to sit down.
Grab end of belt in left hand and pull laterally (and slightly upward) across legs exerting enough tension so that holster/gun doesn't flop or go lower than partition. You have to provide a little lateral pressure with your legs to counteract the pressure you are putting on the belt with your hand.
If I'm going to be there a while I sometimes will buckle the belt back up using the last belt hole. That keeps things loose enough to do your thing but provides enough tension to keep the holster/gun up.
I'm sure I'm making it sound more complicated than it is.