First, wait at least a year after any tickets. If you don't have any tickets, look to switch as your insurance is up for renewal - your current insurer will likely give a great deal to keep you.
Switching liability insurance is usually easiest at renewal. This being said, the best time may be when it comes to mind as most people do not review their policies on a regular basis. Also note: it never hurts to shop around.
You can't switch liability until after your automobile is paid off. Beware. Liability insurance is generally just "collision" insurance, if your car is stolen, you won't get anything.