What size tank do you have? Guppies are pretty easy to breed, but can also be sometimes stubborn.
Keep your water parameters in good condition. I do 25% water changes at least twice a week. I also feed a wide variety of food such as spirulina, bloodworms, brine shrimp, seaweed. I keep the temp in my tank at 78 or 80.
Make sure your tank has plenty of plants for cover without crowding the fish.
Talk to them at least twice a day. I place my hand on the side of the tank and talk in a soft voice. Don't know if this really helps, but I like it.
Remember that guppies breed quickly. However, you may not have any fry for a month or more. And, sometimes they will not breed at all. If this happens, try and find the cause of why they aren't breeding. I found once that when I had a clown pleco in with them, they wouldn't breed, but once I took him out, I had one batch of fry. Good luck