You admit that your previous writing credentials probably helped you (a lot). OTOH, I have nothing to "show and tell" and am stumped as to how to overcome that obstacle. Any thoughts?
I do have a few thoughts, which I'll offer, but since this isn't my area of expertise, I tapped the brain of my venerable agent, who kindly put forth a few tips as well. But, since you asked, here are my thoughts: yes, my magazine experience probably garnered my query a closer look, but, BUT, once I shot off my partial (or full), it didn't help me for a damn. If the agents didn't like what they read, they couldn't have cared less if I'd been the editor of the New Yorker. So, with that in mind, regardless of your experience or day job or lack of publishing credits, you need to write a kick-ass query, which means you need to avoid using overwrought cliches (Miss Snark has posted on the ones that drive her up the wall - search her archives), spell-check it (yes, you'd be amazed at how many people don't), and most critically, write a finely honed, engaging book, one that grabs the agent's attention from the very first page. (See above as to how I axed the first five chapters. Everything that happened before chapter six could easily be summed up in the pages that followed.)
Now, my brilliant agent's thoughts: "Yes, all of what you said is great. (AWS: she's talking about my advice.) I would add that they do their homework. When I get a submission that says, for example, 'I know you recently sold Allison Winn Scotch’s novel, and mine is in a similar vein, …or 'I know you are looking for female memoirs,' that makes a difference. The internet has made it a lot easier for people to find out what agents are looking for or have sold, and I, for one, pay more attention to a blind query that comes in with some thought to whom they’ve sent it. A lot of this is common sense, but you'd be amazed as to how few people do it."