Make a mini-outline. Think of the topics you want to cover, then go back and expand on them.
When I applied, I had a ridiculous number of secondaries so I had to set up a system. On the first night I'd outline topics for each prompt, then I'd go back and expand on them until I developed a complete answer. Then the next day I'd edit for spelling/grammar/content and send. It was sort of like an assembly line, and it kept me on point.