I can NEVER get my sheet cakes to stay in one piece when transferring them to the cake board. Even if I use multiple cakes to make it into one sheet cake.

Any suggestions?

You can handle them slightly frozen or always keep them on a board that's slightly smaller.

Sometimes, I have had that problem too. What I do is after they have completely cooled on a rack and you are ready to either freeze them or put them on a board, put them back in the pan that you baked them in(you have already cleaned it) and then put a cake board over the pan and flip the board over holding the pan to it so the cake does not break or go anywhere, but the board. then either freeze so you can handle whatever else you need to do with the cake or ice or whatever. I have one cake friend that freezes her cakes in the pan so you could do that after it has completely cooled as well and wrap it with double layer saran wrap and heavy duty aluminum foil. I put a piece of saran wrap in my pan first and then put the cake in it that way you can easily get it out of the pan when taking it out of the freezer. I tried it once and it works great. I didn't have an immediate need for the pan before I needed to freeze the cake and take it out. My friend is in the wedding cake business so has lots of pans and can afford to do this. Usually I just put them on a cake board the size of the cake and wrap in saran wrap twice and then heavy duty foil.

I hope this helps you some.