I grew up as the youngest child, and, as my older sisters will tell you (and probably any older siblings would confirm about their youngest siblings), this means I got away with the most. This "most" behavior was never anything horrible; I wasn't a miscreant by any stretch of the imagination. It just means that I stayed out later and took my parents' car out more. No one blinked an eye when I dyed my hair or shaved my head (a story for another time) or decided that camouflage mini-skirts were my new favorite thing. In fact, the only hard and fast rule from my parents that I can remember about my appearance had to do with my confidence. If I looked uncomfortable--rutching my skirt down or slumping my shoulders or crossing my arms over my chest--my parents would suggest that I change my clothes. After all, what we see when we look at a person is, at first glance, his or her clothes, but the next thing that we see and the impression that we carry with us is the person's confidence.
Now, you've probably read a lot of agents complain about the queries from authors we get boasting, "This book is the next Harry Potter. The next Da Vinci Code. The next Twilight. The bestseller that will make your career." Most certainly, do not be that author. But don't be obsequious either. "I know you have better things to do than read my query. I grovel at the feet of your greatness. I'm sure this isn't up to your standards, but here's my query anyways."
You've worked hard on your manuscript and, presumably, you believe that it's publishable. (If you don't believe that, you need to go back and revise, revise again.) Present your manuscript without any apologies--"I know you're probably up to your ears in queries. Another vampire story, I know."--and let the agent draw his or her own conclusion. After all, you need an agent who values your work, and no one's going to value your work if you don't.
So, chin up. Before you query next time or pitch an agent at a conference, hold your mother's voice in your head (or my mother's, for that matter): "Watch your posture, dear." Or, if that doesn't do it for you, how about Leonard Bernstein's Jets? "Walk tall! We always walk tall! We're Jets! The greatest!"