What can u do to prevent blood clots while on your period?

Reduce frequency. You doctor can prescribe you birth control medication that would limit your period to once a year or so. If you do not wish to do that take oral iron and multivitamins to prevent anemia and consult your gynecologist to ascertain the cause of heavy bleeding.
Depends. If you have heavy flow with clots some thing is irritating the lining ( endometrium ), like fibroids, displaced iud etc, see your gyn doctor for cause and treatment will prevent it.