Samantha Clark, who chimed in on yesterday's conversation, wrote a fabulous post today called "Preparation is Key." She reminds writers that we generally only have one shot with each agent (or agency as it may be) for each project (an intimidating notion if you have a short list of agents), and gives great advice on making sure your manuscript is ready and that you've done the proper research.
Towards the bottom of the post, Samantha admits she previously sent out work that wasn't ready and was "roundly rejected." This happens to A LOT of writers, and for many, that's the way they learn to write a better book, to take the process seriously, and to do the research they didn't do before.
I can't guarantee you'll get published by following "the rules" - this business is subjective and relies on writing a publishable book, after all - and I can't guarantee you won't have to write another book (or three) to develop the required skills to do so, but you might be able to save yourself some time, rejection, and grief by stepping up your game and removing any and all avoidable mistakes from your journey to publication. Like querying before you're really ready.
Check out Samantha's blog, Day By Day Writer, and make sure to wish her the best on her newly well-prepared-for search for representation.