Not sure without seeing the resulting geometry but I would try using the merge command .
If you are using the mirror command (shift - V), make sure you set the merge points check box in the numeric panel. and be sure you mirror with the points on the edge matched over the original points, sometimes you can be off just a tad and the points won't merge and you get some wonkiness.
If you use the mirror (x,y, or z) command in the multiply tab, then it should automatically mirror and weld the points at the same time.
If you mirror and the object is not symmetrical first, then you might have problems. Always on the center x axis d a select half and set value to 0 click OK, then delete the selected polys and then mirror.
If you are talking about mirroring say an arm or leg or the like over to the other side. be sure you select all the polys involved first in wireframe mode. If you don't you might not get what you wanted.
Hope that helps a bit
