High reps, heavy weight within reason, get a pump and hold a big stretch. If you are doing it right it should be grueling, they about the slowest twitch muscle you have and are remarkable strong.
A couple of my favorites are big 40 rep (5x8) dropsets on the seated, and 10+5+5 reps on a leg press while holding the bottom position in a big stretch for a solid 10 secs in between reps.
As someone fairly small who has never carried alot of excess poundage it took alot of work to bring my calves up, they looking pretty good now.
And don't forget tibialis training, this can help get a good round look to your calf. The tibialis is the muscle that allows dorsi flexion of the foot, like a toe raise.