The quickest way to mask a foul smell coming out of your mouth is by chewing gum. There are tons of flavors on the market, so it's easy to find one that you like. I have always found strong peppermint gums to be the best. The scent of the peppermint oil does a great job at covering up any unwanted smell.
Chew on parsley
Ever notice that many Italian food restaurants include a sprig of parsley with a meal? Most people move it to the side, thinking that it's just for decoration. If you actually chew on the parsley, you can negate any bad breath forming. It works especially well for foods that are high in garlic. Some people think that parsley also helps indigestion, so go ahead and kill two birds with one stone.
Gargle with mouthwash
Instead of masking the odor, kill it with mouthwash. These rinses contain ingredients that kill germs and bacteria - the bad breath culprits. It's very easy to swish around some mouthwash for a minute before the bad breath gets out of control. Be careful not to swallow it, however.
Most times, bad breath is caused by the foods we eat and bacteria that grows in our mouths. If you have chronic stinky breath, however, it may be the sign of an underlying medical issue. See your doctor or dentist if your bad breath persists.