On YouTube I found a video on how to professionally clean a SLR camera lense. I did not try it, yet. So if you want to give it a try do it on your own risk.
Video on YouTube: http://www.youtube.com/watch?v=G7S6VARidHk
I solved the problem by adding an UV filter in front of the objective lens. So if something gets dirty it is the filter and not the lense. In case you can remove the filter and wash it with warm water and liquid soap or washing up.
Any comments on that?