Sometimes, disease-causing organisms hitch a ride into your aquarium on plastic aquarium plants. If you're restarting a tank after a disease outbreak, or if you get decorations from an unknown source, you should sterilize them. You can do this several ways, but each method has pros and cons.
Boiling
Boiling is one of the oldest methods of sterilization. To do this, get a pot large enough to hold your plastic plants and raise the water to a boil. Add the plants and keep them boiling for at least 15 minutes. This may not be sufficient to kill every single microorganism but it will take care of the vast majority of disease-causing organisms. Keep in mind that this process may melt some aquarium plants or cause them to fade.
Bleaching
You can use bleach to sterilize plants, but you must never do this in the aquarium itself. To bleach plastic plants, add them to a bucket and add chlorine to the water until you can smell it. Let the plants soak for at least 24 hours in a well-ventilated area. Chlorine binds up into noncaustic compounds when it's exposed to organic matter, so if you stop smelling chlorine during this 24-hour period, add more until you can. After 24 hours, rinse the plants, drain the water, and fill the bucket with new water. Add aquarium dechlorinator in at least five times the standard dosage. You can't add too much dechlorinator -- and it's cheap -- so be liberal in your use of it. Let the plants stand in the bucket of dechlorinator water for at least 24 hours, then repeat at least once. If you still smell chlorine, repeat until you can't.
Cleaning New Plants
New plastic aquarium plants do not require full-on sterilization. The procedure for preparing them is shorter and simpler. All you have to do is rinse them in cold water for about five minutes. This removes any dust from the factory or the pet shop and avoids the off chance of clouding the water.
A Word of Caution
Sometimes you don't want to sterilize aquarium plants. In aquariums, beneficial bacteria eat fish waste, controlling ammonia and nitrite levels. So if your plastic plants came from from a tank with healthy fish, you shouldn't sterilize the plants. Adding "dirty" plants can actually improve the health of your aquarium. Since boiling or bleaching can damage or fade plastic plants, you may want to just buy new plants. Unless you don't know where your plants came from or you've had an outbreak of disease, you're better off just using plastic plants without sterilizing them.