Whether or not we're querying or published, agented or un-agented, we've all received some good advice at one time or another on the long and laborious road to publishing, sometimes bad advice too, but let's stick to the good stuff.
I think the most important piece of advice I've received to date is that writing trumps all. It doesn't matter how often you social network, post on your blog, join new forums or market yourself, if the writing is not there, it's simply not there. In other words, stop dwelling on everything else and worry only about the writing. If you are throwing yourself into your work, making it as compelling and captivating as it can be, working out the plot holes, uplifting your characters, the other things have a way of working themselves out. My agent told me this about six months back and she is so right. She says she's seen careers tank because a writer is worrying about everything but their writing. Now, whenever those nasty voices creep in, telling me I'm not doing enough, those significant words she said talk well over them, "Only worry about your writing. The rest will come."