Not that I'm an expert, but here's what I've figured out:
Yes, you have to submit, submit, submit. And think about your layouts in the following way:
1. What will look good when it's reduced to 5x5?
(or whatever the size that it will be in the magazine.) Is the journaling legible at that size?
2. Is there a perspective that I can offer that's unique?
They're going to get a thousand Christmas layouts. How is your story different? Or your artistic style?
3. Is it reproducible?
This is the hardest one for me. Magazines want to inspire not intimidate their readers. Also, multi-photo layouts are very desirable for this reason.
4. Are the photos appropriate and crisp and clear?
Yes, Ali Edwards gets published with blurry photos. But, the rest of us just don't. And try to avoid photos that feature people with their shirts off or with inappropriate words on their shirts. Remember, magazines make their money from advertisers.
And finally, I'll just add that the best thing I ever did was learn about design principles. Even if it's instinctual to you, it's a good idea.
What I didn't say, and I have been thinking about, is figuring out why you want to get published. If it's because you want to be famous or make money or just see your name in print, then I say more power to you. Rock on and go for it! But, if it's for some kind of affirmation of your talent, then I say tread carefully. Magazines are looking for a very specific kind of layout with a very specific kind of look. Just because it's not your style, doesn't mean that you're not talented. So, just keep that in mind when you're submitting your masterwork and not hearing back. It's not about you. It's about what they're looking for.
I hope that's helpful!