Don't leave coursework until the last minute. You're unlikely to get a good grade if you rush it, and you won't be able to make any alterations to it if after you've given it in you remember a mistake you made.
The earlier you complete your coursework, the more time you'll have to improve it. You'll also be able to give your teacher a better chance to look over your draft work so he/she can suggest any changes you could make.
If you're doing your coursework on a computer, make sure you save it at regular intervals while you're working on it, and always keep a back-up copy on floppy disc.
If you're writing your coursework out by hand and you don't have a copy of it, make sure you keep all your rough work. That way, if your neat work gets lost or damaged, it'll be much easier to rewrite it. If possible, the best thing to do is to photocopy your work and keep the copy in a safe place, perhaps in a separate folder labelled 'back-up copies'.
It's worth putting in some real time and effort on your coursework. At GCSE level, coursework can be worth up to 60% of your grade (in science it's usually between 25-50%). Remember, the better your coursework is, the less pressure there will be on you in the final exam.
Click below for a brief guide on how to write up a piece of coursework!