(Frankly, it's better to get an email that says your work is "unreadable, unmarketable, and unpublishable," than a bland "Your story does not fit into our list, right now." At least you get to work up some righteous indignation.)
1. Perseverance. Be stubborn and refuse to ever give up.
2. Work. Cultivate a constant desire to improve as a writer. This means putting words, lots and lots of words, on paper. This also involves studying the craft of writing and reading as much as you can as often as possible.
3. Teflon Mentality. Develop a force field to deflect ego-smashing rejections. It is crucial to have the ability to effectively handle rejection, letting them bounce off you, and not allowing them to stop you from plugging away. (See #2 Work.)
4. Patience. Here’s a little secret — the world of publishing runs on a completely alternate universe concept of time. Tired of waiting to hear back from an agent or publisher or editor? Grab a beer and put your feet up.If you keep all of the above in mind, and, in Ray Bradbury's words, "Write like hell!" I'm sure you can beat my record. (Although, you may not beat C. S. Lewis'. Click here for his truly astonishing number of rejections.)
What this means for you, as a writer who does not want to go fifty shades of gray waiting for an agent to call, is that you can do both. If you've given it all you've got, don't grab a beer - design a cover, and publish.