When Dirt Bikes are new they generally come with fixed brake and clutch levers, there are at least three strategies you can employ:
Carry spares with you that you can replace in times of emergency. Part of your spares kit that you carry with you should have a set of replacement clutch and brake levers so when you hit the dirt and brake a lever at least you can replace it.
Replace your levers with pivot levers.
These levers will replace your original ones and will bend forward and sideways so the chance of actually breaking a lever is greatly reduced. See you local Bike shop and get advice. Having a set of pivot levers is almost essential
Install Bark Busters
You can also fit a good set of barkbusters as the barkbusters usually will take the punishment instead of your levers.
Another way to avoid broken levers is to ensure that the cluth and brake perches are not rigidly attached to the bars, just loosen them enough to ensure that they move if hit, this absorbs the impact better and could just save or maybe bend it instead of snapping it off.