Make the prompt age appropriate. Consider that various age groups understand and interpret instruction quite differently. Middle-schoolers generally cannot approach a prompt with the same level of clarity that a high school or college student can, for example. Perform a role reversal exercise and have students teach the class or you about what it is they think the prompt requires. Allow kids to do this task either individually or in groups. Let them use lists on the classroom board, too.
Stick to a certain portion of the prompt. Do this if you want students to be particularly specific or narrow in their responses. Understand that there may be a myriad of ways a kid may interpret what it is you tell him. Require classroom planning time. Write down three ways you'd anticipate how a group of students may respond to a prompt, then they can narrow down their ideas about how they want to move on from there. Do this alone or have kids do it in pairs. Narrow down concepts or topics and have each of the children anticipate how their neighbor may answer a prompt, too.
Teach students about internal dialogue. Help students organize their thoughts, prioritize ideas and weed out any confusion or anxiety that they may have with a prompt this way. Assign a task where students simply talk to themselves as if they are two people. Have them do this at home or in the classroom. Give students the option to playact with others if they do not feel that they can perform the internal dialogue task on their own.
Use visual aids. Have students answer a prompt in stages with flashcards, pictures and magazine graphics, for example. Require students be as minimal as possible using one card, photo or poster board. Opt to draw a tree with various branches on either a chalk or whiteboard with the class as a whole. Link ideas between prompt concepts using leaves or even animals for starters.