I worked on spin serves last year. Kick, slice, top-slice. Also vary the pace.
If you go back a few years into the archives, i spent a few years tryin' to learn the kick serve to no avail (when i first joined this forum). Also, tried to learn multiple serves at once. Then i changed course, and only worked on one serve for a couple years. Flat at first. Then one spin serve. Now multiple spin serves. Brain gets confused if you're learning all the serves at once and it was a total fail.
If you go back a few years into the archives, i spent a few years tryin' to learn the kick serve to no avail (when i first joined this forum). Also, tried to learn multiple serves at once. Then i changed course, and only worked on one serve for a couple years. Flat at first. Then one spin serve. Now multiple spin serves. Brain gets confused if you're learning all the serves at once and it was a total fail.