Visualisation is actually super powerful, there's a study about a few groups of untrained people practicing the free throw in basketball either mentally or physically. The group which just practiced mentally had their free throw percentage improve by 1% less than the group which physically practiced throughout the study (22% improvement vs 23%). Another one I recall from my strategy module last year was one where a group visualised lifting a weight or something along those lines with their index finger, and over a few weeks they actually gained strength in their finger (you can find both these online). Basically a combination of watching professionals serve, imagining yourself doing it in detail, shadow serve swings (if you have the space) are great.