When writing The Letter to agents and publishers, do it when you're in the right frame of mind. Don't do it when you're sad, when your dog just died, or your bowels are irritated; don't write it when you feel down about life and where you're going (or not), or when you get home from a lousy date.
Write these missives when you feel good about who you are, or when in that odd mood that strikes us at times when we just don't give a damn about "the rules" and write with our real personality. I have found that THOSE letters are the ones that get noticed and have people calling. Mood makes such a difference in how we approach things like this.
I experienced this with a client of mine. He sent me a query to review and it sucked. Why? Not because he can't do it, but because I could tell he wasn't in that letter, not the real him, the real him that could talk someone down from a ledge. I told him to get back to it when he felt ready. He emailed back saying I was 100% right, that he'd had a rough summer and was getting down on the whole publishing thing. He took my advice and rewrote it. Two weeks later I got a great query letter from him.
When selling yourself and your ideas, make sure you are ready to do a terrific job highlighting the best parts of you and your work. Have someone you know read it and ask them if it sounds like you. Don't be overly formal or over-the-top funny. Find a good balance between who you are and the rules of query writing that should be followed.