Rule:
Before abandoning some effort already underway, consider how much resources (time, money, political capital) you have already invested.

Aphorisms:
Don't change horses in the middle of the stream.
There ain't no such thing as a free lunch.

See Sunk costs for the economic critique of this rule. Your decision to continue a course of action should only depend on the current Value of doing so, and not on how much you have invested. Yet in a world that is pathologically unpredictable, nobody knows what the future value will be (least of all economists.) Valuable results usually only come from effort — though our efforts may be wasted, given that we cannot know the true value, we use the amount we have invested as a proxy.