Hi Ramon - yes like Burr mentions when you draw the curves for your shape you want to pay special attention to the first 2 and last 2 control points - the line between the ending 2 control points is what defines the direction of the curve tangent and so if you're going to mirror the results later you want to make sure the curves you are constructing things with have a horizontal tangent.

If you're still having difficulty maybe you can post your 3DM model file so we can take a look at your model.

Also depending on how you are building the surfaces it can sometimes be good to build a larger surface all in one go instead of building half and mirroring it - see here for some examples:

http://moi3d.com/forum/index.php?webtag=MOI&msg=1398.18
http://moi3d.com/forum/index.php?webtag=MOI&msg=1398.19


The other ways that you can get smoothness is by leaving some space between objects and using the Blend command which puts in a smooth connector surface in between the space between 2 edges, or by building things with some initially sharp corners and then using fillets to smooth them out.

- Michael