Don't ask a professional translator to translate into a language that is not his or her mother tongue. Even if he or she could do it, at a pinch, the quality will not be the same as that provided by a native speaker.
Make sure you give translators all the background documents they might need (such as similar documents translated earlier, technical documentation and glossaries). This will make for a better end result. It will also save your time, because the translator will not need to ask you so many basic questions.
Always specify the purpose of the translation. (Is it just for information? Or will it be published? Will It be used for advertising?) If a translation is going to be published, your quality requirements will obviously be higher and the translator will have to check the translation more carefully. If a rough translation has been provided just for information, you should always consult the translator before using it for any other purpose.
Allow enough time for translation. Remember that good translators have many clients — you are not the only one. Don't expect to get 100 pages translated in three days! The technicality of a document and the difficulty of the original language will affect the time required for translation.
Give feedback on the translation. Then the next job the translator does for you will be even better!