Whatever states of mind, happy or unhappy, occur, never mind — we should constantly be reminding ourselves, "This is uncertain."
This is something people don't consider very much, that "this is uncertain." Just this is the vital factor that will bring about wisdom. In order to cease our coming and going and find rest, we only need to say, "this is uncertain." Sometimes we may be distraught over something to the point that tears are flowing; that is something not certain. When moods of desire or aversion come to us, we should just remind ourselves of this one thing. Whether standing, walking, sitting, or lying down, whatever appears is uncertain. Can't you do this? Keep it up no matter what happens. Give it a try. You don't need a lot — just this will work. This is something that brings wisdom.
The way I practice meditation is not very complicated — just this. This is what it all comes down to: "It's uncertain." Everything meets at this point.
You don't need to keep track of all the various instances of mental experience. When you sit in meditation, there may be various conditions of mind appearing, seeing and knowing all manner of things, experiencing different states. Don't keep track of them, and don't get wrapped up in them. You only need to remind yourself that they're uncertain. That's enough. That's simple, and it's easy to do. Then you can stop. Knowledge will come, but then don't make too much out of that or get attached to it.
This understanding of things is always timely and relevant. At all times, it is impermanence that rules. This is something you should meditate on.
— Ajahn Chah in Everything Arises, Everything Falls Away by Ajahn Chah, Paul Breiter