Now, I don't know on whether or not there's an easier way to do this, but first create a new folder somewhere. Add a picture of the skin (if you have one) by copying it (into the new folder) and renaming it to "pack.png." Next, write a .txt about the skin. Name it "pack.txt." Make sure they're both in the folder you've created. Make another new folder inside the one you've already made, and name it "mob." Put a copy of the skin in the folder. Name it "char.png." Next, go back to the last folder with the two "pack.x"'s. Select all and add them to a zip archive, and name it to whatever you want. Put this in the texturepacks folder, and when you play choose a texturepack. Should work fine, and maintains all the normal game textures. If you want to change the skin in a texturepack, just open the packs mob folder with winrar or 7zip or something. Select "add files to archive," by right clicking and replace the char.png with one of your own.