There are a few tips:
1, turn them up-side-down at night. The gravity will help it get longer.
2, due the thermo expansion, coral will be 0.04% longer for every degree incease in temperature. For a frag 4 inch in length in 80F, it will be 4.04 inch if you increase the water to 105F.
3, dose those "joint juice" from Costco. You know, the one that help old people with joint problem. I usually just get the sample, and they wonder why a man in early 30's want to sample it.
4, trim the side branches for Acropora and branching Montipora. This will get the hormone to concentrate on the tip,.
5, put a light bulb behind a fan. As the fan rotates, it makes the light to appear on and off. This will fool the coral to think that a few centuries has passed (one light/dark cycle is one day), and you can see the growth. Remove the hood since some acropora are know to knock off the light canopy.