Please check the teatime-mag from time to time  - if you can, take a look at the exercises as well.

http://www.teatime-mag.com/de/

One Word a Day is also still very recommendable.  http://owad.de/

There's also Business-owad, if you want to try it: http://www.owad-for-business.de/

If you want to spend some time, here's a good homepage with a lot of material!

There are also a lot of pdfs for you to study and repeat - with solutions!

I found a great source with a lot of information and help for you - please take some time and check this out.

I know, it's an awful lot, but if you have a specific problem, you can search for it there and read the article, maybe it's the solution!

There are some free samples available at PONS - check it out.

You may want to try these pdf documents (PONS) as well.