This is something I’m seeing more and more – the rule of 12. In the last week, I’ve seen words to this effect on several websites, and once in a book I picked up at the library, and now I’m going to put them here.
Serious writers will have several submissions out at all times. This sounds really difficult but it means that you have to keep writing. Don’t write one manuscript only and think that’s enough and wait for something to happen with it before doing anything else. Keep writing. Work on the next idea, and the next, and the next. Once you’ve got a number of submissions out, then you must keep proper track of them and if you do receive a rejection, ensure you already have another publisher/agent lined up so that there’s a 24 to 48 hour turnaround getting another submission out again.
Get the manuscripts out there but be sure to study the market and only send them to places that are looking for material along the lines of what you’ve written. Don’t waste your time or theirs by sending a fantasy story to a publisher who only wants horror.