Ace/Roc is seeing too many urban fantasy submissions and not enough military SF, says editor Ginjer Buchanan. She has some great advice on getting an agent/editor, including do your research first. And write a synopsis that explains what actually happens.
"be familiar with the genre that you want to write in"
I'm not sure that this actually IS good advice. I'd think that something like, "don't think of your work in terms of genres" would actually be MORE helpful.
Encouraging young authors to sit down and say, "Okay, military science fiction, what is military science fiction like? David Weber, he writes military science fiction, let me read a lot of David Weber. Good, I'm going to write me some military science fiction," is a fancy way of saying, "Let's get ourselves a whole bunch of David Weber clones!"