1. Proofread. Frankly, if I could only give one piece of advice, "proofread" would be it. Don't rely on Microsoft Word to spell-check for you, either.
2. Check your firm research. It's no good going on about their recent expansion into Mongolia if the Ulan Bator office has since been shut down.
3. Check that your application is specific enough to that firm. "Friendly atmosphere" and "work hard, play hard culture" could be anything from the Harlequins rugby team to Pret a Manger. Find something unique about the firm you're applying for.
4. Consider whether you need to get in touch with the firm before you submit. Poor grades that will get your application binned? Overseas qualifications that don't quite fit in the form? Pick up the phone to the firm you're applying for and explain your circumstances. Yes, it's scary, but it means that they'll be looking for your application and have your explanation to hand.
5. ... but don't make excuses. If you have an explanation for poor grades or gaps in your work experience, make sure it's as objective and factual as possible.
6. Mention any dates that you won't be available for interview in your cover note (if applicable). You'll only look more organised.
7. Make sure your work experience is relevant to the role. Have you explained how that stint behind the bar in Wetherspoons made you work well under pressure?
8. Check that you've entered your grades accurately. There's little worse than being invited to interview (or offered a job) and then needing to explain that that 'A' in maths should have read 'C'.
9. Re-read the application guidelines to make sure you're doing what's asked. Believe it or not, some firms still want hand-written cover letters.
10. Have a break if you can, then read your cover letter and answers to any long-form questions out loud. Does anything sound clunky? Does it need re-writing in shorter sentences?