The success of any translator depends on the quality of his or her work.  In this article, Marion Schimmelpfennig lists 10 steps to deliver high quality translations:

• Accept only jobs that fall into your area of expertise. Do I have to say more?
• If you believe you need more time for the job than the deadline allows, ask for a longer deadline up front and explain why: that quality takes time. This usually works (except for real rush jobs), because often the client does have a bit of a leeway.
• Especially when you are working with marketing material, ask for the marketing brief (target group, marketing objective, layout, media …) or other important information like possible space limitations in the final product.
• Always activate the spell and grammar checker of your software so that you can instantly recognize possible errors. Tip: If the software does not point out ANY errors, chances are good you haven’t activated it at all because almost every text contains proper names or highly specialized terms that your software does not know.
• If you are really stuck with a term or sentence, move on to the next one to avoid frustration and time constraints, but don’t forget to highlight it!
• Do not translate if you are even a tiny bit unsure. Those wrongly translated sentences always look logical when seen alone, but they don’t really make sense or fit the context. Use tools like Linguee.com to avoid that – they deliver terms in sample translations which makes it much easier to determine the real meaning.
• Talking about editing: If you know that a second translator is going to edit your translation, don’t fall into the trap of doing a half-hearted job, simply because you know that someone will find all the errors anyway.
• Disaster check: Depending into which language you translate, the volume of the target copy is always a certain percentage higher or lower than the source copy. Do a character count – if the result does not match your experience, you should check whether you forgot to translate something or maybe even double-translated something.
• Proof-reading: PRINT YOUR TRANSLATION. This is much safer than reading the copy on your computer screen. Especially look for errors your software cannot find. In English, this would be terms like then and than, or were and where. Read your translation out loud. This is more efficient because you can find errors more easily, and you can also check the flow of your sentences. Pronounce EACH SYLLABLE. You’ll be surprised how many typos you will find this way.
• And last but not least: Check the formatting of your file. Does it have to comply with any guidelines? If not, make sure it looks professional and is easy to read.

See: Copy Writing