Understand What Business Services are Needed (and don't be a "Sally the Skier")
When you first meet someone new, whether introduced through a mutual friend or someone you happen to meet while you're out at a social function, it would be peculiar to you if the conversation went like this:
"Sally, this is David. He is the guy who recently moved here that I was telling you about, and he also works at home like me."
"Nevermind any of that. First of all, I've been a skier for 15 years, and can ski any black slope there is. So if you ever want to go skiing, I'll be way better than anyone else, and you'll want me as your teacher..."
Not only would this be ridiculous and annoyingly self-absorbed, but Sally has not even considered whether David gives a hoot about skiing or that she is a pro skier - maybe he has no interest in skiing. This same approach is true when you are composing a proposal for a buyer. Just like submitting a cover letter or a resume objective is often required for application when you find a job, so is an introduction to a project proposal needed - like a kind of mini-objective or statement of purpose on a job resume or cover letter, for online evaluation. The proposal introduction should communicate that you understand the buyer's needs, and what business services he is seeking. It should not jump right into how or why you are the best one for the job.
Identify with the Buyer or Customer
While there is no one right way to write an introduction, it does not have to be long or involved. Just be sure to gradually transition to the body of the proposal with an appropriate introduction. Here's a brief example:
Hello, and thank you for posting the job - Judging from your description, you need a French translator who has had experience translating technical engineering documents, and one who also thoroughly understands differences between English and French cultures.
Here's a brief example of a bad introduction or proposal start:
Hi. I'm extremely qualified for this project because I just finished an engineering manual translation from English to French for a Parisian distributor.
While the second introduction may be appropriate in tone, it does not address the fact that you have read and understood what it is that the buyer or customer needs. Rushing right in by assuming that your qualifications match what the buyer is looking for may be off-putting to him, especially if you have misunderstood or wrongly assumed the most important factor and/or qualification to the buyer/customer for hiring a service provider.
Focus on the Customer/Buyer - Not Yourself
Make sure your introduction is polite and concise, but mostly importantly, acknowledge that you understand the buyer's need - whether it is technical French translation, or legal consulting for liability clauses. The primary purpose of the introduction should always be focused upon the buyer and what business services he needs - not what a good service provider you are, or why you are most qualified.
If you stick to this basic approach, more often than not you'll get off on the right foot with your buyer. Avoid being a "Sally the Skier" in your intro. Any regular person would want to throw their glass of water on someone like her.