It is important to vary the structure of your written work to make it more interesting. Here are three ways that you can do it:
1) Use a mixture of simple, compound and complex sentences.
It was raining. (simple)
It was raining but he didn't mind. (compound)
He went outside although it was raining. (complex)
2) Use a wide range of connectives to join and link your sentences.
however, although, additionally, whereas, so, but, if
3) Add meaning, depth and detail to your sentences through your choice of vocabulary and word classes.
Reluctantly, he got out his umbrella and approached the rainy street.
In this worksheet you can read some extracts from students' work and use the above information to decide what their targets for improvement could be.