Check all your text with a spell checker before it is published. Read your own work over and over for grammatical correctness and logical sense. Think about how you can make the written message more effective whilst using fewer words. Everyone appreciates brevity. Then get somebody else (with a good English writing background) to proof read it for you, even if it costs you a few bucks, a cup of coffee or whatever. Truly, it will be money well spent.
Also don’t ignore differences in American English vs British English. Choose one form of the language and then stick to it consistently.
For commercial sites you should even consider hiring a professional copy writer to craft your text into a compelling form. If English is not your first language, consider this mandatory. Content writing is a vastly underestimated skill, yet well written content is another essential element in lifting your site up to a more professional standard. At the very least though, don’t shoot yourself in the foot with sloppy attention to detail.