There is absolutely never a reason to assist feed an animal that has ever eaten in the past. If the snake is losing large amounts of weight due to a fast then there is something wrong. Also, you may want to rethink your weight requirements. I have a Spider male that has sired about 15 clutches over the years, is 4 years old, and has never weighed more than 780 grams.
Mitch,
In response to your post, I have seen some good explanations in here, but have seen one thing left out that we do. We let the prey sit in a cup of water for a few seconds to get wet. i believe this helps to lubricate the food before attempting to push it into the mouth. The only other hint I can give, is that we generally push the mouse into the snake until the front shoulders have just disappeared into the mouth of the snake. I have seen some people on this thread state that as soon as it gets into the mouth, it triggers a feeding response. It simply isn't that easy in most cases. We use the above method, as sometimes it is required to hit the throat before the feeding response kicks in. Even doing that, I have had animals assisted for 6 feedings before they get the hang of it.
Hope that helps,