Imagery, or mental rehearsal, is one of the most used tools in sport psychology. Research generally supports it, especially when combined with physical practice.
What makes imagery more effective, according to common models:
Make it vivid and multisensory: what you see, hear and feel, including muscle sensations
Match the real situation as closely as possible: the venue, the time, the pressure, even what you would wear
Rehearse at roughly real speed for skills, not only in slow motion
Include handling problems, such as a bad start or a mistake, and recovering from them
A short routine to try: 5 minutes the night before and on the morning of a race, rehearsing the start, one key moment in the middle, and the finish. Picture yourself executing calmly, including how your body feels.
Regular short sessions tend to work better than one long session.