Ignition switch lockup

Every once in a while, more often than not, the ignition switch on my 2002 Toyota Camry will not turn. The instruction materials say to rock the steering wheel side to side, I do but it seems to take a while and it is tough since the steering wheel is locked too. It is so unpredictable as to when it will refuse to turn I am always wondering if it is going to occur every time I turn the ignitiion off. What can I do to prevent this or to predict when it will happen? I travel alone mostly or with my elderly mom and this gives me a lack of cofidence in this car that I like in all other ways.