The longer the session is, the more I'll shoot. If I edge for 3-4 hours I'll shoot a massive load. I drink lots of water while I'm jacking and that seems to help but may just be in my mind.
I shoot the hardest and farthest if I fully grip my shaft just before I cum and pull down toward my body so that my head is stretched really tight.
The harder my cock is also makes a difference in how far I shoot. When I was younger I was always hard as granite everytime I jacked off and my load would come out like a rocket. Now that I'm older and have better control I can cum with a half hard cock if I want to or fully hard.
The hornier I am helps the strength of the spurt too.
The other morning I was jacking hard before work, still horny as hell from meeting a new hot chat slut the night before. Normally when I cum these days I'll shoot up onto my chest and stomach, sometimes reaching my neck. Well that morning I shot so hard that the first huge spurt hit me square in the face with a huge splatter.