On the Absolute Write forums, a writer suggested that even a questionable or amateur agent might be able to "get a clean query letter in front of an editorial assistant who reads all of the first five sentences of Chapter One before passing judgement".
Much of the resulting discussion focused on possible pitfalls of dealing with such agents, but I wanted to comment further about the first five sentences. It's quite possible that this is all the editorial assistants read - not only because they have stacks of manuscripts, partials or sample pages to get through, but because one bite is usually enough to tell whether you're eating an apple pie or a mud pie. As an example of first paragraphs that are a turnoff, check out this collection from POD-dy Mouth. My favorite is, "They called her Labia."
I've seen similar opening sentences in a lot more books and in manuscripts presented for critiques, so a good way to both stand out from the crowd and hold the editorial assistant's attention is to hone the start. Begin with a character in trouble, an unusual setting, a vivid image, a hint of danger.
The moss-ball bounced across the floor of the cavern towards Fernfeather. He lunged forward to sweep it aside, but stumbled at the crucial moment. It seemed to mock him as it rolled slowly past his outstretched leg and into the goal.
Roger Eldridge, The Shadow of the Gloom-World
Some places are too evil to be allowed to exist. Some cities are too wicked to be suffered. Calcutta is such a place.
Dan Simmons, Song of Kali
It's easier to work a gripping hook into the first five sentences (maybe even the first sentence) than to deal with an inexperienced agent.