"If a fish can fit into the mouth of another, it probably will." :(

Single species tanks seem to have the best fry survival rates. Maybe that is because using olfactory senses (smell, taste, feel) fish can tell the difference between "our fry" and "lunch" (their fry).

What size aquarium is your pair of guppies in? What are the tank mates, if any? What kind of filtration is there, if any? What kind of hiding places (live plants, plastic plants, rocks, Lockrocks, plastic sunken ships, other "structure") might be in that tank?

A way to keep even cannibalistic livebearers from eating their fry is to put a lot of fast growing plants (hard water favorites such as hornwort, Najas or even water sprite will work well) into the tank. If possible, only the pair should be left in there. In a couple of cases with cannibalistic species (though even certain pairs may be different in "personality") there were so many plants packed in there, that the adult fish could hardly swim.

Then, without over feeding the parents (easier said than done), gradually feed them some formerly frozen food, a little bit at a time, until the adults are so full that they are almost "waddling". That is prepared by taking some bloodworms (don't touch them and wipe your eyes) or glassworms or even brine shrimp and defrosting them in Luke-warm tap water. When defrosted, they are then rinsed through a sieve or fine-meshed net to get rid of that "soup" accompanying the frozen food and then the net is emptied back into the jar with some clean water you had treated and let sit a couple of days (preparing to use it in water changes). A lot of aquarists break down and buy a good turkey baster (or get a pippet) which is only used for aquarium purposes.

(Ages ago my wife and I realized that she didn't want fishy dirt on her kitchen ware and I didn't want killer soapy stuff anywhere near the fish. So we got several items for the kitchen and several of the same items for use with the fish.) For some ideas for a Christmas in July (or December)see
http://www.guppylog.com/story/2003/11/17/10939/076

Guppies and an amazing number of livebearers (and even some egglayers) will leave their fry alone so long as there are other meaty foods they can eat instead of the fry. In nature and in the aquarium, cannibalism takes place when the fish are too crowded or there isn't enough food to go around. But virtually always, if there is an alternative food, the fry will be ignored.

We don't want to over feed much because rotting meat in an aquarium is bad news. A couple small pond snails, such as the ram's horn snails, are useful in cleaning up extra food.

Sigh, and if you are feeding heavily, you will need to increase the number of partial (40%?) water changes you are doing on your tank, maybe beyond that often mentioned goal of weekly, partial water changes.

(By the way, if you have a population explosion of snails, you have been overfeeding. It is better to have to take out some extra snails than have your tank get really cloudy and funky from the bacterial explosion caused by rotting food or than to have your fish die of some nasty disease caused by the poor water quality, which cripples their immune systems.)

Sometimes I'm in the position to buy a portion or two of those live black worms at my LFS (Live Fish Shop). They are rinsed in a wide-mouthed gallon jar until all dead worms are washed out. Then they are stored in blue plastic worm holders.
http://www.aquaticfoods.com/accessories.html

About an ounce of worms can be stored in each worm holder. Those holders slide under our refrigerator's cold-cut drawer and no-one else needs to even see them in the refrigerator. Of course I have cleared this with my bride first. ;)

In several of our aquariums, a clean (soap-less) jar (salsa, jelly, mustard, pickle...) is placed where I can easily reach in with a turkey baster and leave some blackworms. Maybe 20 of those worms are fed at a time. (I don't put much salt in the water because it will kill those worms and cause the whole tank to crash.) Dry foods (regular flake, algae flake and even marine flakes) are fed earlier in the day.

Sometimes we're lucky enough to have some Daphnia, which can be squirted into the aquarium (if there is no powerful filter to suck them out). Those crustaceans, which form an important part of food chains all over the world, will cruise around the aquarium, feeding upon algae and bacteria in the water, until they are eaten. In a case or two enough Daphnia was put into the tank so that the breeding pair couldn't eat them all. The Daphnia continued to swim around and even drop baby Daphnia! That was great. The adult fish ate the Daphnia rather than the fry. The fry ate the baby Daphnia! That is almost fishbreeder heaven. ;)

The above are examples of what are sometimes called "natural set-ups." If you Google that term, you will probably get a lot of hits. There is also a great natural aquarium web site by the lady who writes a plant column for TFH (Tropical Fish Hobbyist) Magazine.
http://naturalaquariums.com/

I have snatched 25 fry from fish (the red-tailed Goodied or Gambusia or Brachyraphis) who never before left us any surviving fry before the adults were sated with meaty foods. Guppylog members, when feeding "expecting" guppies or swordtails, have been astonished by how many more fry they found than with previous drops.

Hope this wasn't too much at one time. (If it wasn't, also read through the Immediate Help stuff, especially sections 2-8 here. Click on the link in the upper right hand corner.)

Please also consider the questions about your aquarium. Is there anything which you might be able to do to help insure the survival of fry?