If at any time you have the opportunity to attend specialist classes, these may of course be very useful. This page offers a series of tentative recommendations for self-directed learning of scientific English. In all cases, a central element will be the study of articles from your field. If you can afford it, buy a big dictionary.
Specialist dictionaries are of course also useful.
There are very few books dealing with scientific English for non-natives. However, it is a very good book, and much of the language it covers will be relevant to practising researchers, particularly if your level of English is relatively low. This book deals specifically with writing scientific research reports, and is written specifically for people who are not native speakers. But in general, probably the most useful text currently available. It considers all aspects of the writing process, including for example choice of journal.