Guaranteeing quality

There are various essential processes for ensuring translation quality:

Research
Using the correct terminology is, clearly, crucial to translation quality; finding it, however, is difficult and time consuming. Which is why it sometimes falls by the wayside when budgets are tight or time is short. I research all specialist terms with which I am not already intimately familiar using relevant specialist dictionaries, journals, any reference materials provided, the internet and where necessary by discussing it with contacts in the relevant field.

Feedback
In practice, texts frequently contain points, the precise meaning of which, though clear enough to the author, may be less apparent to the reader. A rough understanding which may be sufficient for the intended audience is not sufficient for a translator. Where clarification is required, you will receive a list of points for elucidation where the deadline allows or comments where it does not.

Quality control
All translations are subject to the following quality control processes:

bilingual proof-reading – checking that the translation accurately renders the content of the original and is complete (nothing missing)
monolingual proof-reading – checking that the translation reads as a well-written English-language document and uses an appropriate style
spellchecking
grammar checking

Where appropriate (e.g. for printed or marketing materials) translations are also proof-read by a second translator.

Getting the best out of your translator

If you've got the time and the inclination, it's well worth reading the ITI's brochure on getting the best out of your translator: Translations - getting it right. A guide to buying translations (PDF).

More succinctly, there are a number of things you can do to help get the best translation:

Clarify the purpose of the translation
A translation for a printed marketing document requires a very natural sounding marketing tone, whereas for a short internal e-mail announcement the important thing may be just to get the message across. The former will naturally take more time to produce and cost more than the latter. Register is also crucial. A medical text written for health care professionals will require a rather different tone to a text written for patients.
Tell me who and what your text is for and I can produce a quality and price to suit your needs.

Allow a sensible amount of time
Translation is not simply a matter of mechanically swapping one word for another, it's a creative process. Try an online translation service such as babelfish and this quickly becomes apparent.
If it's taken you three weeks to write your document, it stands to reason that a translator is not going to be able to produce a foreign language version of that same document overnight.
For the translator, it's not just a question of getting his or her thoughts down on paper. He or she first needs to understand the thoughts of the author, who will usually be intimately familiar with the subject matter. By contrast the translator first needs to work his way into the material and carry out the necessary research.
As a rule of thumb a translator can translate around 1500 words a day.
Sometimes project deadlines mean that it's just not possible to allow adequate time, and it's always possible to produce a rush job, but quality will suffer as a result, so you're helping yourself if you allow adequate time for translation in your original project planning.