Strategies depend on the age and grade, but if we are discussing grammar school-age students, one of the standard techniques is to ask students to keep a daily journal in which they write both the meaningful and mundane. Ask students to write about the worst and best thing that happens to them on any given day if they tell you that nothing happens in their lives to write about. The point is, at least in my view, never to worry about whether they write well, but to get them used to engaging the hand and the mind at the same time. In other words, I think early writing strategies should be designed to create some muscle memory. One of the benefits of students' early and regular writing is that the teacher can often identify students with as-yet unidentified learning problems--not dyslexia, necessarily, but more subtle issues like tracking.
For regular assignments, you might consider assigning short (1/2 to 3/4 page) descriptions. One assignment that works well for grammar and middle school-students is to ask them to describe the room in their home in which they feel the most comfortable, and prepare them for the assignment with a discussion of the elements of description. Once they have the idea of how a description works, you simply keep them writing descriptions. Most of the time, at least some will need appropriate topics assigned.
From description, move to narration and ask students to narrate the events of their day or week, not in informal journal form, but as if they were telling a story to a stranger. A narration, because it is linear (usually), organizes itself. Again, this requires some preparation.