How do you avoid grammatical mistakes while writing an article?

Use technical tools to assist - spellcheck, thesaurus. 

Self-check article at least3 times.

Have someone else read it - a fresh pair of eyes always helps.

If writing in second language, it is particularly important to make sure that the way words are interpreted have been correctly translated to the other language.  This is where alot of grammatical errors can occur.  

Always read article twice and use online grammar checker tools, especially if you have deadline and don`t have enough time to check it twice. 

Always read your article twice and get a colleague to have a look at it

re read the article or show it to a friend , everyone is blind when it comes to their grammer