I think the best advice is just "Don't give up." But then I think that if a person is going to be a writer, that's pretty standard. I mean, you can't do anything if you give up, right? Doy. But that's not what I always think about.
What has always and forever stuck in my mind is not a piece of advice I've gleaned from a book, but a little factoid snatched from the pages of Chicken Soup for the Writer's Soul. Just the fact that Stephen King was rejected over 100 times before self publishing, and J.K. Rowling was turned down almost just as much (if not the same amount). I don't remember the exact numbers. That's not what's important. What is important to me is that authors that famous with work that good had an editor somewhere saying, "Sorry, but this isn't going to sell," or "Sorry, it's not what we're looking for," and that same editor a million copies later thinking to himself, "Wow, am I the biggest idiot ever." Now an editor may stand by his decision, and by all means, he or she is allowed to do so. They made that decision based on what information they had at the time and it just happened to turn out the opposite way. The book selling world is kind of like a storm at sea - it could change at any minute from just a cloud to a huge, whirling hurricane. What might not sell on Monday might be the biggest thing since sliced bread on Tuesday. Editors are just like weather forcasters - they can only predict, they don't know for sure.
What I'm getting at is if big name authors used to get those goofy form letters the same way I do, then in some ways, I'm not much different from them. I tuck them away in a little folder and hope for better things next time. The knowledge that rejection is a part of writing is ingrained in me - I accept it. It still sucks, but hey, you gotta kiss a lot of frogs before you find a prince, right? There are people out there who are going to think their story is the next big thing; their baby all grown up. But then they get rejected from one place and shut down forever. No thanks. I know I'm not perfect, and I know my writing isn't perfect, so I'm going to keep on moving until I see something in print. I don't care how long it takes. It's a very specific "don't give up."
And you know, when I do get published, I can just look at all my rejection letters and smile, and say, "Thanks guys, for helping me to get where I am now."