The Tips in detail

 "How to get rid of opacity-ghosts?"

Applying an opacity-map to a material can result in a somewhat (mostly white or greyish) outlined model. To get rid of this, you should scale the UV-opacity-map (in the material editor) a bit smaler than the map to be shown (eg. 1 x 1 for the diffuse and 0.98 x 0.98 for the opacity).

Another thing to remember is that the texture map should not be anti-aliased against a white background as white is much more present than eg. black.