Why does my cakes always sag after I put fondant on them??? it just looks sloppy! What am I doing wrong?? Help!!!!

Because fondant is heavier than icing, you need to have a sturdier cake to withstand the weight. If you use a box mix, I suggest that you try a cake extender/WASC recipe. If you are a scratch baker, you should look for a recipe where the crumb is a little more dense. HTH

Also using ganache under fondant works well. It firms up better than icing and can hold the weight better. Maybe try rolling your fondant thinner as well.