Not sure if this is a new way or not, but found a way of getting a great fillet to wing seam today. On a TF spit, I ironed down some ultracote where the fuse sits on the wing, wiped it down with wd40 and spread bondo on the underside of the fillet and bolted fuse to wing. Then took more bondo and spread along the fillet/wing seam and let set. Removed fuse after the bond set and sanded smooth. Great fit and easy. I left the bondo edge slight heavy so it's not so fragile, but it could be sanded down to nothing.
What other techniques do you guys use?

shaping blue/pink foam & glass seems too work well also.

a modified method of doing it is to make the part line at about 80% down the fillet to the wing. The joint becomes verticle and is much less delicate. You can achieve a perfect feather into the wing, and the wing is positively located when attaching it to the fueslage. Here's the result on my TF Spitfire. You can even plan it at a panel line.

Even better!