quick tip for sealing the end of San Juan Chenille is to lightly burn the end of the material.
If you're not doing this already, then it will help, but fact is, chenille is not a very durable material for this purpose. But, it's what most folks use, because it seems to have the best action. Using "ultra" type chenilles, which are denser than regular chenille helps too.
Chenilles are fine for wrapping bodies, like on Woolly Buggers, but for free form purposes, they don't hold up well.
I've even tried to add some twist to the chenille, twisting it more tightly in my fingers, then adding a drop of super glue gel near where I want the end to be, then once the glue dries cutting off the excess. This helps too, but they still come unraveled, and you don't get the same affect as the burning.
You can double the chenille & twist it over itself, but that makes a larger diameter worm, not well suited for smaller size hooks. The only other way to make worms more durable, without adding a lot of bulk, is to use another material for the extended section, like chamois and color it with permanent markers. Unfortunately, you won't likely get the color as nice as what you get with chenille!