Immediately after sharing a status or comment on Facebook, you can quickly edit it by clicking the "x" on the post. Sounds contradictory, but the typing field will reappear for you to make the appropriate changes.
Note: If Facebook asks you if you want to delete it, it has been too long and you no longer can edit the post.
A writing tip in general is always "save" what you are writing. This can be done with word processors, but how can you do it online? A simple safety precaution I take, is copying whatever I'm writing, whether its a status, tweet, blog post or comment — especially if its very specific or long. Your fingers are already at the keyboard so just hit "Ctrl+A", then "Ctrl+C". You have now just copied the text to your clipboard.
If you want to go a step further you can paste it in a text field on a website such as Notepad.cc. Now if your webpage crashes, you accidentally delete everything, etc. You haven't lost all your work.