Everything Ember said.
But I can tie my first sale directly to PI. Until I joined, I'd never heard of e-publishers or Ellora's Cave. I was trying to write a book for Blaze, and I wrote 50K on it in less than 7 weeks, but it was too hot and I didn't know where else to try to sell it. I knew there were other things wrong with it that I just couldn't put my finger on, so I tacked a hurried The End on it and filed it away, not to be looked at again for another year. I absolutely HATED it. But I'd accomplished my goal of "finishing" a book, so I treated myself to an RWA membership, and I found PI pretty soon thereafter.
What I learned here set my imagination on fire. I was so excited! I wrote my next book, Alien Overnight, specifically to enter it into the Stroke of Midnight contest. I used the Yahoo crit loop to help hone my entry, and the critiques I received, as well as the crits I did for others, helped me hone my craft even more. The book placed second, and though the editor didn't buy it, she gave me very encouraging feedback. She said she just personally didn't care for sci-fi but she was sure someone else would pick it up. So I entered it in another contest, it won, and the final judge, Heather Osborn, snapped it up six days after I submitted the requested full.
Then I took that first book that I'd written back to the crit loop and got invaluable feedback on it. The ladies there helped me to realize my weaknesses as a writer (ahem, the incredible disappearing plot :wink_smile: ), which is every bit as important as knowing your strengths. I submitted it to Laura Bradford because she was a member here and I felt she'd be less mean to me than others might -- a comforting illusion to cling to, LOL! But she was very nice and told me exactly what wasn't working for her. I made the changes she suggested and submitted it again, but she told me in a handwritten note that she LOVED the characters but the plot didn't set her world on fire -- which was no surprise. I subbed it to another agent who said the plot was appealing but the characterization needed help. :wallbash: Really?!
I disregarded that last agent's comments because every single critter and contest judge (and Laura Bradford!) had told me that the characters were excellent but the plot needed shoring up. I also disregarded the contest judges who said I needed to get rid of the prologue and work the hero's inciting childhood incident in as back story. That's another thing being a member here helped me to do -- learn when to listen to my own gut about my stories. I learned that just because something is a trend in the industry doesn't mean that every trend fits or will improve my story. I tried entering the book in a couple of contests without the prologue and without exception, the judges HATED the hero.
Then I submitted the book to my then-current editor, who said she loved the story and offered me a contract on the condition I write out a certain plot element, which she said readers did not care for in erotic romance. I agonized for days, but ultimately, I had to turn down the contract. I told her I thought eliminating that element would weaken both h/h's character arcs. She was very pleasant about it and told me she was sure I'd have no trouble selling it elsewhere. I would never have had the courage to turn down a contract if I weren't a member of RWA and PI, with the writers here telling me it was MY story and I needed to go with my gut. (Incidentally, that editor left shortly thereafter and my new editor agreed that the story would have been weaker without that plot element. She bought it [with prologue] without a single change.)
Several of the workshops I've taken here have helped me firm up weak areas in my writing. I've learned that I need a visible representation of my books (like a plot board or a bullet list of scenes -- anything that boils it down so I can see the whole book at a glance) to really round them out fully. Shelley Bradley's workshop about storyboarding was a huge EUREKA! moment for me, and it helped me to rescue another manuscript I'd disgorged very quickly and put away for almost a year. (50K in 26 days for NaNoWriMo -- I'll never do THAT again!) There came a certain point where I didn't need it anymore to finish the book, but it really helped me to sort and flesh out all the crap I'd spewed in those 26 days.
So, long story short (NOT), RWA and PI are great places to find education, encouragement and support, which you need just as much after publication as before.