Decide in advance where and when, not whether
The gap between intending and doing closes when you name a time and a place.
Do this
Write your intention in the form: if it is [time] and I am at [place], then I will [specific action]. Not 'exercise more' but 'if it is 7am and I am in the kitchen, I will put my running shoes on'. The trigger must be something you will physically encounter.
Why it works
Called an implementation intention. It shifts control of the behaviour from effortful deliberation to an environmental cue, so you stop spending willpower on the decision. A meta-analysis across 94 studies found a medium-to-large effect on goal attainment, which is unusually strong for something this cheap.
Where it fails
It does nothing for goals you do not actually want. It also fails if the cue is vague ('when I have time') or if the action is too large to start in one step. And the effect shrinks when the behaviour competes with a strong existing habit in the same context.
How you’ll know
Within two weeks you should notice the moment arriving and the action starting before you have consciously argued with yourself about it.