Rule 10: Work on your knowledge of grammar and writing rules, read good books and magazines, use reference tools and edit your documents – especially your emails – after letting them rest a while.
You'll be getting loads of tips on how to improve your writing as we go along. But if I had to shortlist those I think would give you the best returns in the shortest time, I'd give you these five:
2. Read, read, read. That includes reading good newspapers and magazines, but also try to read as much non-fiction as you can on whatever interests you. And good fiction will certainly work too! There are many lists of 'must-read' classics on the Internet – try goodreads for starters. While you're reading, look out for constructions that you're unsure of and re-read them a few times until you get the hang of it.
Example: its / it's
Correct usage: It's time I gave the cat its worm medicine.
3. Use a dictionary and a thesaurus, and even a style manual or two. You're welcome to contact me at email@example.com for the titles of those I'd personally recommend.
4. Let your writing cool off before editing it. You should preferably leave it overnight, but even a coffee break can help at a push.
5. ALWAYS edit your writing before sending it off, even if it's 'just an email'. For really important documents, arrange with someone else to swop out and edit each other's work, if you can't afford to have it professionally edited. (But allow me to quote HG Wells here, though: “No passion in the world is equal to the passion to alter someone else's draft.”)
To sum up: Know your grammar and writing rules, read, use reference tools and edit your cooled-off documents.