WAYS TO FILL LAYERS - To avoid cake filling "overflow" or bulges, build a FROSTING DAM.   

Often, when cake layers are put together and the cake is iced, the frosting will bulge on the outside (showing between the layers). They can develop in a frosted cake when the cake layer settles and has a hump on top that had formed during baking. If a hump does form, with the next cake:

A. Build a frosting dam in between the layers:

    1)  Using a decorating bag with or without a large round tip, pipe a thick icing "dam" just inside the edge of the cake. It will prevent filling from squeezing out when another layer is placed on it and will help level the cake. This will work only as long as the filling is strong enough to hold up the layer placed on it.   

Fill a pastry bag with medium consistency icing and use tip #10 or 12 (nothing bigger) or use the coupler without adding a tip, to pipe with.  

Start with the bottom layer, leveled or flat side up. Squeeze a circle about 3/4- inch high and along the inside edge so filling doesn't form a bubble.  

Fill with enough good-tasting filling to flavor but not enough to cause the layers to slip. Starting at the center, spread the filling of your choice, such as pudding, custard, fruit, etc. inside of the dam with a spoon, knife, or spatula, working your way out toward the edge, creating a nice, thick layer. I have some fillings that I use that are a bit too liquidy so I don't go up to the line. 

Place the next layer on top, making sure it is level. The weight of the layer will cause the circle of icing to expand just right. Even push tiny cake scraps in between if there are large gaps.

Simply press the top layer into the bottom gently, but firmly and check that they are level. You might still need to cut off a sliver or add a small piece of cake to level. Let them set together several hours or overnight. This allows the cake layers to settle so they will not bulge between the layers and make seams. (-Seams- aren’t icing pushing out from between the layers. They happen when cakes weren’t leveled before assembling.)  

Make sure the cake is the same height all the way around. Move two rulers around the cake, positioning them at perpendicular angles, to measure and adjust the cake so that it is the same height everywhere. One ruler rests across the top of the cake, the other will measure the height. You can also just eye the cake to see if it's even. 

    2) Base (crumb) coat the outside before applying the final coat. After base coating the outside of the cake and then let it sit overnight to settle. The next day, if there is a little bulge of icing in between the layers, trim with an offset spatula so it is flush and smooth. Then, apply the final coat of icing. It seems to do the trick! 

B. Other solutions:

For wedding cakes: To eliminate any chance of bulging, I cut my dowels taller than the cake. To do this I decide how many dowels I will use and take the uncut dowel and stick it in each place I will put it. 

Each time, I put it in and place a mark just above the icing, then remove it and repeat for each location. Then I pick which hole was the highest and cut the dowel for that hole just slightly taller than the top of the icing. 

Then I cut all the dowels that exact length. That way, the cake board is not resting at all on the cake, but slightly above. The gap will be covered by your border at the bottom of the cake. I have never had a problem with bulging of the icing and the cake is still very sturdy.

Another way is to cut a circle, square or whatever the shape of your cake about 1/2 inch from the edge. Use an offset spatula to dig out the inside about 1/4 inch deep. Then, fill this with the filling. Again, this only works if your filling has enough thickness to hold up the center of the top layer. 

some information from http://w3.one.net/~proicer/cakepict/icingcakes.htm