There is an automatic mechanism which saves your draft every 45 seconds.
That means you can write your answer and wait 45 seconds before you leave the page, then your draft is saved. When you return to that question you can continue where you left off.
You can find out if it was saved by looking for the draft saved
text below your edit box.
If you need more control of that mechanism then you can use George Edison's awesome Manual draft save scriptManual draft save script.