There is no secret, if you want to have bigger triceps then you need to train all three heads, let me show you how. Starting off with smith machine jam press to train medial and lateral heads of the triceps. Think about bringing the bar down towards your chin while keeping your elbows up and then allow full extension at the top. Next stop dual rope triceps extensions. Arms in line with your torso and 30 degrees out to the side, make sure you extend to a full lockout. This will target the long head of the triceps in a shortened position. Finally, overhead extensions to hit the long head in a lengthened position. Focus on keeping your arms back and push your armpits forward for maximum stretch.