I've had good and bad luck with my labs holding. In my 55g my large female has held at least 3 times that I can think of and I've never to this day seen any of the babies.
In my 20g where I'm growing out juvies, I've had a bunch of females holding and I've tried a couple things.
1. first time, I caught the female and attempted to strip into a floating fry tank. With the one female I did it too early and the fry weren't developed enough. The second time I did this, I ended up with 5 fry which i grew out and are now about 1/2" in with the other juvies (big enough to not get eaten).
2. next time, i caught 2 females that were holding and used a divider in the tank to separate them from the rest of the fish. This worked best in my opinion because then the mothers aren't harrassed and can spit when ready (which both did). I ended up with a 3rd female holding after the first 2 spit and put her in with the fry (counting on the fact that she wouldn't eat the fry babies free swimming ... which she didn't) ... then when the mothers spit, i move them out of the partititioned section (which I'd guess is about 1/5 of the 20g tank). I also threw some extra java fern for the fry to hide in.
I've been feeding my fry the Hikari "First Bites" which have worked well for me. I currently have 13 fry growing like mad. Its fun to watch how fast they grow and see them learn that when I walk over to the tank, food is coming and they all come out of hiding (since the larger fish aren't a threat).
(you can see the babies and java fern on the left and the larger labs in the main section of the tank)