Many people feel that cake filling can be difficult to maintain. If done right, you can make a beautiful filled cake, without too much drama.
Cake filling flavors need to compliment one another, chocolate with orange, hazelnuts, or almonds
Some cakes will not hold up well without refrigeration. Mousse and whipped cream can separate, get soggy and drip. Berries can get soggy and drip everywhere. Buttercream needs to be kept in a cool place so the frosting wont separate. Cakes covered with fondant should be kept at room temperature and never be refrigerated.
If you are using fresh berries or whipped cream as a filling, make sure the cake will be eaten within a few hours. If these things are on the cake too long. If you are going to prepare a cake a couple of days in advance, you may want to use simple syrup or a flavor wash on the cake layers to keep the level of moisture up, before you add your filling.
Keep in mind the dietary restrictions of your guests. Make sure there is no conflict when you are decorating a cake for special occasions, or for a client. If diabetics are present, you might want to make or purchase a small dessert that is sugar free for these guests.
When you are making a cake for an occasion that is serving other food, make your flavors stronger and your colors brighter. This will make the cake stand out and your cake decorating will be something they will remember.