Make sure your grammar is good and makes sense. Try to avoid filler words quality matters way more than quantity in an application. Try to tie in some IRL qualities you have that may benefit in your ability to be staff (example: you've been staff on another server, you take responsibilities in school or in clubs you might be in, etc) Make sure you don't have colored words in your application just to keep it more clean and easy to read. Be honest, that's a given.
Another big one is not to give up.learn from your mistakes and make sure they don't get repeated. They may not tell you the reason you've been declined, but you shouldn't wait for someone to tell you the reason, you should try finding that out yourself first. ^-^