The key is a very good offcontact and a tight screen. That has helped me. Obviously my technique has gotten better since I started, but it really just seems to be an issue of:
-having enough adhesive on your platen
-having the correct off-contact
-tighter screens. The tighter screen(not higher mesh) will pop back up quicker and seem to give a better print.
I do a flood stroke then a print stroke. I repeat the flood then do a print stroke with less pressure and less angle so it's more like spreading peanut butter on bread. that seems to get me decent white on darks.
oh almost forgot... I use anywhere from 86 to 156 mesh screens for white ink. I picked up 86 mesh for puffer ink and underbases, but I still prefer using 110 and up for underbases.