Tools and techniques
How much rain/water is enough?
I notice when my local council plant trees they put a small pipe in the soil adjacent to the root. This allows the water to go through a crust. I guess this can be done with pots too
edited July 2018
The other trick is to water everything at least twice. Go round and give a little bit to let it soak in, then go round again & give a proper drench- otherwise you are right it just runs through. Most propriety potting composts have a wetting agent added to them but things like coir do not this can be very hard to re-wet once very dry. I always use dishes under pots so if it drains through the plant can take it back up in it's own time.
AB Still learning
The simplest way to keep large numbers of pots watered is to stand them in gravel trays. You then just fill, or part fill the tray with water, and the plants draw it up from below. No need to water again until the tray is empty.