How could I avoid the breaking of a shinai?
Usually shinais are made of bamboo. So there ever will be the risk of breaking or splintering during the training. Nevertheless you can increase the lifetime of your shinai if you mind the following:
Before you use the shinai the first time, disassemble it completely. Sharp edges should be rounded off with a smooth sandpaper to avoid splintering.
The single braces should be oiled regularly to keep them flexible. Excessive oil should be removed.
Turn the braces of the shinai after four to five trainings, so that the beating-brace comes to the back of the shinai (or to the side). By turning the braces the risk of breaking will be spreaded over all the four braces.
Always check your shinai before use. If there are any splinters, you must take the responsibility to file them down and fix it!