I've done 120 cum-on-pics videos, always working to get long spurts, and in my experience, the important factors are:

1. Drink lots of water ALL DAY before the shoot.
2. No caffeine. (I'm sorry but it's true -- it prevents hydration).
3. Get your PRE-CUM flowing!
This was super-important! If your tube is lubricated with pre-cum, the cum shoots out, but if it's dry, the cum goes slow, and the first spurt basically acts as the lubrication.