A goal is a wish. A plan with a specific time, place and cue is an instruction, and your brain runs instructions. Across nearly a hundred studies, people who made "if X, then I do Y" plans followed through much more often than people with the same goal and no plan. It's one of the best-replicated findings in behaviour change. Evidence: solid
The cue has to be something that reliably happens: a time, a place, an event. "When I feel like it" is not a cue.
Same theme: Moving
Moving
Lower the bar for the gym
On a day you'd skip, the session is ten minutes. Same time, same place, bag packed the night before.
10 minEvidence: decent
Moving
Move hard three times a week
Run, lift, cycle, a sport. Hard enough to sweat. Three sessions, any length over twenty minutes.
30 min, 3x a weekEvidence: solid
Moving
Rehearse the lift in your head
Injured, travelling, or stuck at a desk: fifteen minutes imagining the exercise, as vividly as you can, muscles included. Most days.
15 minEvidence: decent