I am just getting started in the whole cake decorating thing and the last cake I made started to sag after I stacked it. I don't know what I did wrong! I made sure to use dowls in the bottom layer and thought that a 2 tier cake would be a snap. I put the top tier on and after about an hour what looked like a giant air bubble appeared about half way around the bottom layer. Any tips to keep this from happening again would be greatly appreciated! 
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What kind of icings did you use?
Did you let your cake settle after filling and putting the layers together, before you iced and stacked?
Did you use enough dowls? All around the side and in the middle. Where the dowels strong enough? It seems as though your top cake was definately squashing into the bottom layer. Did your top cake have a sturdy board underneath? Were your dowels long enough so that there was no pressure on the bottom cake. Somebody once said that you should be able to put bricks on top if the dowls and board where sturdy enough.
after I take mine out of the oven, I IMMEDIATELY smoosh them with a flat cookie sheet to push out some of the air, then I refrigerate or freeze the cake (depending on how long until I need it) giving it more time to settle.
I find that most of the sagging comes as air settles out of the cake under the weight of the frosting and decorations.
HTH
I agree with letting it set. I put a dam around the edge, press. Fill, put on next layer, press. Wrap in plastic and let set over night or several hours before I ice them. So far, (knock on wood) we haven't had a problem.
Lots of dowels and keep it in the fridge as much as possible. My teacher told me this and my cakes quit bubbling. I keep my cake in the fridge all the time so it doesn't settle (while I am not working on it of course.) All cakes will begin to settle eventually left out at room temp. It does not get dry because it should be covered with icing and fondant rather quickly and that keeps the moisture in. Good luck! I hate that bubble!!!!
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