If your toilet is consistently overflowing, you most likely have one of two problems; either the drain is fully or partially clogged, which does not allow the water and sewage to flush sufficiently, or your floater is set to allow too much water to enter the tank before and after flushing. Checking both potential problems is the surest way to prevent future overflow accidents.
Turn off the valve that supplies water to the toilet.
Use a plunger to clear the drainage pipe. Be sure to do this before flushing the toilet so that you force water rather than air into the clog.
Insert the spring-end of a toilet auger into the drain, and rotate it until the full auger has been inserted. Turn the handle a few more times to loosen any clogged material.
Remove the auger and flush the toilet. If your problem was poor drainage, this should clear most clogs. However, if your toilet continues to drain slowly, you may need to consult a professional plumber.
Remove the lid from the toilet tank. Be sure to set it somewhere else as the porcelain can break easily.
Locate the toilet floater. In older models, this is usually a large rubber ball on the end of a plastic or metal stick. In newer models the floater is a movable column that fits around the main toilet tube in the bowl.
Find the mechanism that controls the height of the floater. In the older bulbs, this is often just a small clip that holds the bar attached to the ball. With newer models, you will find small switches on the movable portion of the toilet column that can be adjusted.
Set this mechanism so that the toilet bulb or floater rests lower in the water. With bulbs, this means making sure that the bar angles slightly lower in the water. With the newer floaters, you should move the switch one or two clicks lower so that the floater's resting position is lower on the column.
Flush the toilet, and test the floater's position. You should notice that the water in the tank stops filling before the water line reaches its previous position. If it still allows too much water to enter the tank, lower the floater again until the water level is adequate for your toilet bowl.
Some floater mechanisms can become defective over time and may need to be completely replaced.