Something also to be aware of is that on the occasion a female livebearing fish can choose to re absorb her fry. Also and especially with guppys, sometimes something has gone wrong as the guppy has growen (genetically) and it is actually unable to release or fully gestitate the fry, so gets heavy and then re absorbs, not by choice, but threw nessecity (sp?) This can be a result of poor outcrossing which has left a genetic blunder as far as breeding potency is concerned and can be seen in but not limited to, a lot of albino livebearers.
There are two sure fire ways to increase fry drops, regularity of these drops and health of the fish. IMO NEVER use a breeding trap unless you want to increase your chances of loseing your breeding fish. Have plenty of floating water sprite and well fed fish and you should be fine depending on specie your stocking.
Firstly as already stated, plenty of high qaulity foods, live foods are the best, and through previous dealings i would say specifically, white worms and baby brine shrimp.
Secondly as also previously stated, its all down to your conditions! Plenty of clean fresh water, smaller but more regular water changes, and when it finally comes time that she looks so huge shes going to pop, increase the water temp by a couple degress, then do a large water change and leave over night. Livebearering fish will often release there fry over night and the ignition for this can be the large tempreture swing.
When your fish does release, record the date, expect fish in optimal condition to release a good amount of fry if mature every 4 weeks, at the lastest 6 weeks, sometimes it depends on the specie aswell.