Mix up a preservative solution for your carved pumpkin consisting of 2 teaspoons of household bleach per gallon of water.
Fill a sink, bucket, or tub with enough of the bleach solution to totally immerse the carved jack o lantern. Place the jack o lantern in the bleach mixture right after you have finished carving it. Soak the carved pumpkin for 8 hours or overnight.
Remove the pumpkin from the liquid and allow it to air dry. Spray the pumpkin inside and outside with a commercial pumpkin preservative or use your own mixture, consisting of 1 teaspoon of bleach in water. Spray the pumpkin once daily, to prevent the growth of bacteria and mold.
Smear petroleum jelly on all of the cut surfaces of the pumpkin. This will prevent the pumpkin from drying out and getting that puckered, shriveled look.
Protect the jack o lantern from sun or rain, since one will dry the pumpkin out, while the other will promote the growth of mold. If possible, refrigerate your jack o lantern when it's not in use.
How Pumpkin Preservation Works
Bleach is dilute sodium hypochlorite, an oxidizer that kills microorganism that decay the pumpkin, including mold, fungi, and bacteria. You need to reapply it because it loses its effectiveness fairly quickly. The petroleum jelly locks in moisture so the jack o lantern doesn't get dehydrated.
Now that you know how to keep it fresh, make a science Halloween jack o lantern.