Encouraging ESL writing can be arduous. Here are a few things that may (or may not) help depending on the student(s).

Writing prompted from pictures...

For any level:

http://edgalaxy.com/journal/2010/10/18/150-amazing-images-to-help-students-write-and-think-more-cre.html

Miscellaneous:

http://www.tomslatin.com/80-journal-writing-prompts/

http://esl.fis.edu/learners/writing/misc/journal.htm

One of my favorites is "dialogue journals" where the student and teacher communicate through writing about the chosen subjects for the journal entries. The teacher is more of a facilitator and instigator rather than an active responder. Basically, the teacher doesn't write too much about themselves unless prompted by the student. Also, make sure the student knows you are not correcting their grammar or spelling for this exercise. The exercise is primarily to encourage critical thinking, flow thinking, and habit.

http://www.cal.org/caela/esl_resources/digests/dialogue_journals.html

Teaching Tolerance: A Project of the Southern Poverty Law Center 

This site has important discussions about immigration, social justice, and using writing as a means of not only cultural poesis but community/communication infrastructure.

For the advanced student:

http://www.tolerance.org/classroom-resources