Simplistic answer is to get bigger, stronger, quicker, younger, taller.
However, those options aren't for every tennis player, or you'd see ATP pro looking like Hulk on steriods.
Look at Nishikori, Kohlshreiber, Meltzer, Hewitt, Ferrer, Ferrero, and notice there is an upper limit constrait due to physical HEIGHT. That upper limit is somewhere around the 130's.
Only taller guys can serve faster and expect to get some IN.
130 is fast enough not to have any real world implications.
Federer has one of the best serves ever and he doesn't often break 125.