Being convincing to an employer in Germany

German employers are pretty thorough in their approach to selecting a candidate.
So make sure your application letter and CV make a good impression, are well-worded, and free of any grammar or typing mistakes - these would throw you out of the candidate pool immediately!
Also, and this is true for the cover letter as well as the interview(s), it is expected that you convince the employer of you being the best person for the job. Just writing (or saying) that you are interested will not suffice, you must state in detail why you are motivated and properly prepared for it.
CVs in Germany always have a picture of the applicant - and it must be a nice picture. There are photo studios specialised in CV pictures, so go and visit one!
Traditionally, applications also included copies of educational certificates and evaluation letters from all former employers, but this is currently changing, so it is now acceptable to submit them at a later stage - but submit you must, as these are very important in Germany (you might want to contact all your previous employers for such letters before you start applying).
If you write in any other language, you must state your knowledge level of German (the official classification is A1, A2, B1, B2, C1, C2 and in most cases B2 or at least B1 is needed to find a job). Be prepared to be asked questions in German during the interview to test this!
Job interviews are usually formal affairs at the company's office. You are expected to come on time (meaning not a minute late!) and in jacket and tie. Bring printouts or your CV for all participants, even if you have already submitted it previously.
During the interview, you must not only present yourself well, but also show that you have done your homework about the company, by asking specific questions and pointing out areas you could contribute well.
Some interviewers purposely ask difficult and sometimes awkward questions to test how a candidate behaves under stress. Do not overreact or get angry if you want the job!
Most companies conduct several (minimum two) rounds of interviews, with varying attendants, before a decision is made. Do not talk about salary expectations or contract details before you are specifically asked for it (by HR in a later round of interviews)!
The decision making process is not taken lightly by German companies and it typically takes a few months, so be patient and continue to show your sincere interest.

German employers are very picky, well, I guess one could just say that Germans are picky, but this begs the question, where does this trait come from? The bottom line with German employers is that you have to do whatever they say, impress them, etc...  they want obedient workers. Very rarely do they want an employee who brings something new to the table, although I have seen this on occasion. If you are a foreigner, please pay extra attention to these things about Germans. Do not attempt to bring any of your cultural tidbits into their fold right away as this will be frowned upon. Bottom line. Obey and attempt to be exactly what they are looking for. Don't rock the boat. Just fit in and do as you are told. This is what they want. Perhaps later you can introduce whatever new ideas you might have, but be careful. Germans are set in their ways and they fear most new things. IMO of course.