kevinm,
I run into that problem a lot, it's caused by cutting the sprue too soon and not necessarily from the mould being too hot.I run my melt at full temp to get as much frosting on the bullet as possiable, it seems to hold a bit more lube in the rough surfaces.
I get smears when i get in a hurry and don't let the sprue puddle get firm enough. All the suggestions given here work very well, I use 0000 steel wool on a warm mold.