I can't recommend any "magic" trick that might boast production of terpenes or resin, but the most obvious thing is to keep the environmental parameters and nutrient availability to your plant throughout its flowering period at optimum levels for your plants to live up to their genetic potential. That can mean lots of light, especially spectrum in the further red zone upon flower, but with way more photons it received before the switch to 12/12. A significant increase in light intensity may help a bit with the "stretch" issue as well. But yes, blast them and take them to their optimal VPD level just to the point where they keep their stomata open all day. That will increase "resin production" I guess.
Terpenes are volatile fellows, and will leave your plants faster if the temperature is higher, the humidity lower, and light more intense when growing. But it's really in the dry and cure that terpenes get lost the most in the cycle if you have your growing environment reasonably dialed in. Improving the after-harvest drying and processing of flower is probably the easiest way to getting better at preserving terpenes.
And any site or whatever that promises to influence the trichome density on your calyxes by selling some "organic" liquid thing is likely snake oil IMO.