a) Choose your English: Decide whether you want to use British English or American English. If your customer base is more European it is advisable to use British English. If many of your clients are based in the US, use American English. However, do NOT mix the two languages.
b) Employ a native speaker: You should only get native speakers to write or translate for you. Even if you do not notice it yourself, any native English client/potential client will immediately notice if the writer of your content is not English.
c) Avoid literal translations: It is obvious when a text has been translated from a different language. It often sounds stilted and odd. Ideally, you should create content directly in English. If this is not possible make sure that your translator translates the texts freely to ensure that it sounds colloquial.
d) Cost-cutting: If you are looking to cut costs it is advisable to translate only the key parts of your website, but to do this very well. This is better than translating all of the content cheaply but badly.
e) Proofread. Proofread. Proofread: Always double-check and proof-read your texts before you publish them online. Also remember to do this once they are on the web already. Some parts may have been forgotten whilst uploading. Remember, online content is dynamic and it continuously needs checking and changing.