Hi, i am kind of new to blender, but i understand the basics of modeling and all. So i have been trying to model for so long and there always ends up being some type of error that ruins my product! well i started a new model not too long ago, and it was looking great. It was just a regular human model. I had the mirror modifier on, and had finished the torso neck and legs. i decided i would check to see how it was looking by applying subsurf. It all looked realy great untill i looked at the butt! for some reason the butt that i had made changed when subsurf was applied and now there is a huge butt crack that goes down his butt and all the way back up his belly! i was so frustrated and wanted to know if there is any way to fix this or if i have to start again. (btw it looks fine without subsurf added) 
In the mirror modifier settings enable the ‘clipping’ option so any vertices on the mirrored axis are locked there
Mirror modifier wiki entry: http://wiki.blender.org/index.php/Doc:2.5/Manual/Modifiers/Mesh/Mirror
I did that, but for some reason the weard looking butt crack wont go away. and when i select the verticies and bring them forward it only give me an option to have a protrusion or a crack
Make sure they are really on the mirror axis and you can’t drag them of the axis.
i was able to get rid of the but crack by pulling all the verticies together. but now it seems that there is a line spliting the torso in half
Hello! Before you apply any modifier, make sure that mirror is ABOVE the subsurf…
Dude if you took of the du clipping, there shouldn’t b a problem that occurs like this. You should b able to just move them back where need b. If theres like a huge indented line that divides the torso, select the mesh in edit mode and move it along the x-axis away form the origin point just a little bit. Can’t wait till you get started on the head:evilgrin:
Without a picture I can only assume that you have a bunch of verticies that are not welded along the middle of your object. Clipping would have solved this from the beginning, now you may have to manually weld (merge) them together if you have applied the modifier already. You could try a remove doubles with the verticies selected and increase the threshold until the line disappears, but not so far you collapse your object :).