My buttercream cakes always crack.  Is there a way to prevent that?

are you placing them on inflexible boards? if the board flexes the cake cracks--

You need sturdier cake boards. 

If it still happens you can fix it after it is crusted....dip your spatula in very hot water...

and lightly go over the crack...let the hot water do the work not pressure...it will melt the icing and

fill in the crack.....just a little tip :)

AUse a little corn syrup in your buttercream frosting.

If the board is not thick enough...all the hot water and corn syrup wont help.   It will just crack when it is moved again.     Use a thicker board..........bam........problem solved.   Many people put tierd cakes on the thin white cake boards.   Big cakes or cakes with heavy filings or tiered cakes need to at a minimum be on an 1/2" drum. 

Thanks to all of you.