Practice! Learn how to put a solid essay plan together in 5 minutes. This takes care of the structure and ideas, allowing you to focus on the language and have 5 minutes left to review your essay.

As Jan said, practice. :) How long does it take you to write 250 words? Perhaps you can already write the essay within the time limit. Time yourself first, then work on improving that. Also, work out how many words you write on average, per line. This way, you can anticipate how many lines you'll need as you write. Don't be afraid of crossing out an entire line of text and rewriting it. Make sure you have a plan of how to structure your essay. When you divide it into parts, you'll find that the task is quite easy.