Originally Posted by Prince Planet
You don't. You keep it plain and simple. State that you are responding to the ad, reference your attached resume, your applicable experience, why your interested in the job/why they should take time to meet with you, and that you look forward to hearing from them shortly. The End.
Note: Make sure you proofread it about 100x. While a typo on these boards might result in some mockery, a typo on a cover letter or resume = no job.
Good luck.
p.s.- here' another freebie: 99% of the time, you get hired because you either (1) directly generate profits for the company, or (2) indirectly retain profits for the company by being efficient/reducing risk/cutting costs. That's it. Know which one of those is applicable, and emphasize that point as much as possible without sounding like a tool. [This is during the interview, btw -- not for blathering on in a cover letter.]
p.p.s. - one more: make sure you comply EXACTLY with whatever instructions, if any, are stated in the ad. A demonstrated inability to follow instructions is another surefire way to insulate oneself from any potential interviews.