Please give me some tips to write effectively for exams

I need to know some sentence structures or some tips so that my English writing becomes powerful, effective. I collected some information like below:

• Do not use contractions like don't, can't
• Avoid there is/are
• Use active voice
• Use verbal form if available
• Avoid using empty modifiers like very, a lot, fairly, etc.
• Avoid starting with "It is recommended that or it was found that"

I want to know more. Can you recommended any website or source which can help me?

Thanks