In order to encourage students to write in history class, I often come up with interesting prompts and projects. This project based learning activity I developed to allow students to delve into the primary source material surrounding some escaped slaves. The students read and take notes on the sources they've read. Afterward they write their own stories based on the situations in the runaway slave advertisements they have read.
The project comes complete with a packet of twelve sources and instructions.
Feel free to add your own requirements or adjust mine to suit your needs.
Its also important to note that these sources touch upon some heavy topics like race, racism, and other attitudes of the day. You may also need to interpret some parts of the sources for your students.