I was wondering how you make the filling thicker. Everytime I make a layer cake it smashes the filling down so much that by the time you eat it there isn't a lot of filling. I want it to be thicker but as soon I the next layer goes on it starts coming out the side. How do I make it thicker? If I put the filling on the bottom layer and freeze it than ad the next layer would that work?? Argh, please help.

You need to make a buttercream dam before you put the filling on. Use a big round tip to make a ring of stiff buttercream about 1/2" from the edge of the layer, all the way around. Then put your filling inside and put the next layer on top. Some people mix cake crumbs with their BC to make the dam. I've never made it that stiff but the most runny filling I've ever used was pudding.

Freezing won't help because when the cake thaws, the filling will still ooze out.

I have used the BC dam before and still seems to not be that thick. Maybe I need a thicker dam.

you need a cake dam. There are times when I've had to build mine up pretty high to hold my filling but the stiffer you can make the icing the better it will hold up for you.

If a nice stiff dam still isn't doing it for you, try "cake spackle". You can search for it in the forums here but it's what Texas_Rose alluded to above: buttercream with ground up cake scraps mixed in.