Advice by your supervisor, books, workshops 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).
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 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).