Alone, on a turbo trainer, consistently and progressively. 90 mins max per session.
Plus a weekly chaingang or some sort of circuit training (as in training on your bike, on a crit circuit or a simulated crit circuit, not pushups and whatnot) once a week to work on bunch skills and cornering. Not needed for a time triallist really, I don't do it (and it shows, my bike handling is dire vs that of a road racer or crit racer), but would be imo pretty much critical for a road racer or crit racer since bike handling goes a long way, as does knowing where to stick your front wheel and when.