When you undergo a root canal procedure, the pulp within the affected tooth is removed. This is because the decay within that tooth is so deep that the pulp is diseased and damaged. If left untreated, it can easily result in an infection, which leads to extraction of the tooth. Sometimes, after a procedure has been performed to remedy this problem, an infection still sets in, and you'll need to treat it so that it can heal.
Contact your dentist if you're suffering from a persistent root canal infection. The infection could have caused an abscess that must be medically drained before any other form of treatment can start to work.
Get a prescription for an antibiotic to treat the root canal infection internally. The most effective way to do this is by using a prescription antibiotic that can "kill" the bacteria causing the infection.
Rinse your mouth with a saline (saltwater) solution each morning and night.
Continue good dental hygiene. Though you may be suffering from some pain due the root canal infection, you should continue your normal (or above normal) dental hygiene regimen.
Treat any pain or discomfort due to the root canal infection. Take an anti-inflammatory, like ibuprofen or acetaminophen, to reduce inflammation and lessen pain.
Go in for an additional procedure. Since the root of a tooth can be quite extensive, you may need to go back in to remove any pulp missed from the first procedure or go through an "orthograde" root filling to prevent additional infections.