Good luck!

Sonication will always produce DNA breakage. To what extent, depends on a number of factors (power of the sonication probe, how much you sonicate the sample, etc). You may be able to reach a useful compromise if you do not require high molecular weight DNA, but in any case you will have to proceed by trial and error.

Unfortunately, Alejandro is correct - ultrasonication almost always causes DNA breakage.  If you want to perform probe sonication without nearly as much DNA damage, try using a rotor-stator instead of a sonicator. 

You can find more information on rotor-stator homogenization here: http://homogenizers.net/pages/ac-rotor-stator-homogenization