Your striking bodyparts should be properly conditioned before you attempt to break to avoid injury. You should do conditioning exercises 2-3 times a week on non-consecutive days.
Do pressups on a hard surface (concrete or wood) on your knuckles (NOTE: this shouldn't be done by people under 16). Do the pressups in sets of 20. If you are already doing other chest exercises in a strength training programme, don't do the pressups, but kneel down and put all of your bodyweight on the knuckles.
To make sure only the first two knuckles are toughened, only have these knuckles in contact with the floor and with your weight on them. When you finish the pressups, there should only be marks on the first two knuckles.
This exercise will also help produce a stronger punch as it produces a strong chest, strong triceps and strong shoulders.
This exercise can be used for any striking point (including the knuckles). It simply involves striking or kicking the Makiwara (or striking board). Do Ten reps with each striking point (again, no more than 3 times a week). Gradually build up the force behind the strikes so that on the first, you are hitting lightly and by number 8 or 9, you are hitting as hard as you can without experiencing pain.
This exercise will not only toughen striking points, but it will improve the accuracy of your strikes and allow you to get used to the impact of striking something with full power.