If a toilet seat is slipping--that is, if it shifts suddenly when you sit on it--the seat is either the wrong shape for the bowl or not properly secured.
Toilet bowls come in two basic shapes: round and elongated (or extended). Round bowls measure 16 1/2 inches from the seat-mounting holes at the rear of the bowl to the front. Elongated bowls are 18 1/2 inches.
If your toilet seat hangs over the front of the bowl by more than a fraction of an inch, you have an elongated seat on a round bowl. Replace it with a round seat.
If your seat is the proper size, it may be slipping because the mounting screws have come loose. If the seat wiggles when you tug on it, this is the case.
On most toilet seats the tops of the mounting screws are hidden in compartments attached to the toilet hinges. Pop open the compartment lids and tighten the screws with a screwdriver. You may need to use a wrench to hold the nut under the lid while you tighten. Snap the lids back down.
A seat may also slip because it's broken and needs to be replaced. Check the seat thoroughly for cracks.