Advice by your supervisor, books (https://web.archive.org/web/20120611092138/http://network.nature.com/blogs/user/mfenner/2007/09/30/books-about-scientific-writing?), workshops (https://web.archive.org/web/20120611092138/http://network.nature.com/blogs/user/mfenner/2007/11/26/can-a-workshop-improve-your-scientific-writing?) and a lot of experience can improve the quality of your scientific writing. But when you are about to submit your paper and don't want to take any chances – especially when English is not your first language – a scientific writer can be helpful. You can pay someone to do this, but even better would be a scientific writer that is employed by your university or research organization (I've written about this before (https://web.archive.org/web/20120611092138/http://network.nature.com/topics/show/567?page=2?)).
Not many institutions in Germany have the resources (or vision) to hire such a person. I was therefore glad to see a job posting for a scientific writer at the Max Planck Institute for Infection Biology (https://web.archive.org/web/20120611092138/http://www.mpiib-berlin.mpg.de/?) in Berlin, Germany. More institutions, including of course my own university, should follow this example. (Disclaimer: I'm not affiliated with the Max Planck Institute in any way. I worked across the street for five years).