In most cases, the register and language of the ad will indicate the right style for your letter. Most often, you'll find yourself using a rather traditional and business-like style but for some creative jobs, a creative and lively language might be more appropriate.
Some important points:
Don't be submissive, don't be dominant
It is important to make a confident impression. The quote below is too insecure.
I would be overjoyed if you would happen to find my application interesting.
Never beg. Never apologise. A cover letter has no room for modesty or for bragging. Arrogance can kill a letter, as shown in the following example.
When you'll meet me for the interview, you will wonder how you've ever managed without a dynamic colleague such as myself.
A statement like this is simply insulting for an organisation.
If you want to make a positive impression, you should write a positive letter. Avoid all negativity, including words which may have a negative connotation.
This letter is merely able to unveil a hint of my personality.
I am looking forward to an interview so we might meet in a more personal way.
Also remain positive if you are changing jobs.
I dislike my current occupation.
I am convinced that your company will offer many more opportunities.
Both in style and in argumentation, you should be tactful, something which is clearly not the case in the example below.
I would like to discuss my application with you during an interview. Would that be possible any time soon?
Be brief, write fluently
A cover letter should not be any longer than one page (350 – 400 words). Do not use long sentences or difficult words that you would not normally use in a conversation.
It can be useful to picture the recruiter while writing. What would you say? How would you say it?
Avoid hollow phrases and use your own words. But keep it simple and spontaneous. Do not try to be funny or desperately original.