There is substantial research that suggests taking a moderate asset allocation combined with annual rebalancing will have a fairly predictable return over the long run (a decade or more).
If you're making trades frequently, you're probably (as shown by research) throwing that money away.
There are always things that go up, and others that go down, but a well constructed portfolio will do more of the former and less of the latter. Annual rebalancing then buys the cheaper asset (by definition, on a relative basis, the cheaper asset is bought and the expensive asset sold when rebalancing).
And yes, it is actually that simple. The problem is (as someone else on this thread had posted): fear and greed. If you can ignore that, you can do well.
Greed and ego.