I just picked up this tip and now I wish I had found this out sooner. I'm so excited
On the picture's layer...
1) Ctrl-A
2) Ctrl-X
3) Q
4) Ctrl-V
5) Q
6) Ctrl-Shift-I
7) Edit > Fill > "Black"
And now you should have a whiteless layer of lineart. What can I say? I dislike multiply