what do you use to support the overall weight of a cake (especially a stacked cake)? I know you can use the SPS or whatever stacking method for the tiers, but what do you put under the whole cake? Just doesn't seem like a piece of cardboard or foam core would be sturdy enough. Thanks 
I also use plywood. I have a round piece and a rectangle that I cut. I glued little feet on them so they are easy to pick up and I, of course, cover them each time I use them.
I do this as well. It works great. I even cover it with contact paper depending on the color of the cake.
make sure you put the feet on it like was mentioned above. That is a must 
leah_s, what are drums? sorry, I'm a little new.
Also, plywood sounds great! Do you all get these back or do you have to make new ones for each cake?
So leah, the drums are just cardboard? They support the weight of a 3 tier cake well? Thanks!
I use plywood most of the time as well. I usually cover it in paper to match the cake or sometimes just freezer paper with the shiney side up. Sometimes I cover the plywood in fondant.
I used to ask for the plywood back but then my husband just cut a bunch in different sizes for me so I don't have to worry about it. But that "feet" idea? Definately going to have to remember that! Thank you all for that tip 
I have used drums too. They are much sturdier than just plain cardboard so they'll support your cake no problem. That's what they're made for.
I also like the idea of ceramic tiles. I may have to try that sometime. I love all the information I get from CC. You all are great.
Also, plywood sounds great! Do you all get these back or do you have to make new ones for each cake?
If you want your plywood back to reuse, charge a deposit for it or there's no incentive for anyone to return it. It's also helpful to have your business name and phone number permanently marked on the reverse side along with a message "Please return for deposit" or similar, so if it accidentally gets thrown out at a venue, hopefully someone will notice that it wasn't intended for the trash.
I mostly use drums, but have used plywood in the past (esp. squares) and also the masonite ones you can get -- here's a link
http://www.globalsugarart.com/cat.php?cid=873&s=30&name=Cake%20Boards,%20Boxes,%20Doiles,%20Foil%20&%20Ruffle/Masonite%20Cake%20Boards
to make them taller I either use card board rounds or foam board and cover the whole thing with contact paper.
Thanks everyone! You all are so helpful!
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