Weight painting bones!

I’ve made a low poly character wich I want to rig and use for animation or games. Now’s the problem I rigged it but when I try weight painting like in this tutorial http://mediawiki.blender.org/index.php/BSoD/Introduction_to_Character_Animation/Upper_body_weight_painting I also got the rigging technic from that, but it doesn’t paint the same for shoulder L as I paint for should R if I use X mirror :S It paints what should be for both shoulder L and shoulder R for shoulder R! Hope u get what I mean. If someone could say a site for hosting I could host a blend file or rar so u could see what I mean.