The most important rule to writing effectively is to not repeat yourself. Each of these three rules focuses on avoiding repetition in English.
写作最重要的原则就是不要重复。下面这三点能够教你有效避免重复：

RULE 1: DON'T REPEAT THE SAME WORD
 一、不要重复同一个单词

One of the most important rules in writing English is to avoid repetition. In other words, don't use the same words over and over again. Use synonyms, phrases with a similar meaning, and so on to 'spice up' your writing stile.
英语写作最忌重复。不要翻来覆去地重复一个单词。可以使用同义词、近义短语等，来调节你的写作风格。

RULE 2: DON'T REPEAT THE SAME SENTENCE STYLE
二、不要重复相同的句式

In a similar way, using the same sentence structure by repeating the same structure over and over again is also considered bad style.It's important to know a variety of ways to make the same statement.
句子结构上的单调重复也容易造成写作失分。因此，平时有必要积累相同内容的多样化表达。

Here's a quick review of these forms:
Coordinating Conjunctions
比如使用并列连接词，but, so，for等等。

Subordinating Conjunctions
比如使用逻辑联结词引导一个从句，though等等。

Conjunctive Adverbs
句间副词短语，如as a result,however,otherwise等等。
Prepositions
介词短语。

RULE 3: VARY SEQUENCING AND LINKING LANGUAGE
三、连接词多样化

Finally, when writing longer passages you'll be using linking wordsand sequencing to connect your ideas. As in word choice and sentence style, it's important to vary the linking language you use.
组织比较长的段落的时候，使用连接词、顺序词组织观点。在遣词造句上，尽可能使连接词多样化。
举个例子：

Instead of writing:
First, open the box. Next, take out the equipment. Next, insert the batteries. Next, turn the device on and begin work.
You could write:
First, open the box. Next, take out the equipment. After that, insert the batteries. Finally, turn the device on and begin work.