Act as if you are not surprised. When your child curses do not overreact. When parents get upset, angry, shocked, or perhaps laugh when their child uses profanity it can teach the child to repeat the behavior for attention.
Speak calmly. Always speak in a calm and low voice when you tell your kids that they have done something wrong, especially with cursing. If your child finds that using those words causes you to get upset they will do it just to see if your reaction is the same.
Tell your kids what words are okay and which ones aren't. It is important to let children know what their boundaries are, especially young children. Young children will constantly test their boundaries as it is a part of how they learn about the world. If some words are okay but only in a certain context, like "butt" for example, make sure your child understands the difference.
Invent some new words. One of the best ways to keep kids from cursing is to invent some fun new fake profanities as replacements. This can even turn into a fun activity for you and your child as you both decide how you'd like to spell and use each new silly made-up word.
Enforce at every turn. Don't fail to let your children know when they have said a word that is unacceptable. Let them know immediately and briefly, simply saying something like "That's a bad word and you know it. I do not want to hear that again." Be firm but do not show anger or any other extreme emotion, especially shock.
Make a cussing chart. Using a ruler or straight edge, make some boxes for each day and label them in pen. For every day that your youngster goes without using profanities of any kind let him add a sticker to this special chart that you can hang on the wall or in the kitchen. At the end of the week or month, reward your kid with a special treat.