This is very common and can be disastrous. Obviously the first thing to say is do try to be careful, alert and vigilant at all times. Try to avoid being distracted while mowing, that you are alert and in control and make sure you are aware of where the cable is while you are working.
There are a number of other ways to avoid this. It can often happen because you have a number of trees, branches, beds and borders and it is easy to get tangled up from time to time. You could try buying a model with a cable tidy, this will both prevent stretching and snagging and keep the cable from under your blades.
Perhaps, in the long run, you should think about a cordless electric or petrol driven model, particularly if you have a larger area to mow and lots of hazards to tangle your cable up. Borders and trees may not be the main reason for running over your own cable, but the freedom from the cable means that’s one less thing to think about when you are negotiating your prize begonia bed or a sleeping dog on the lawn.