Writing Tip #1:
Write what you know. Know what you write.
Rather than trying to 'Write what you know, Why not just Know what you write?
You don't HAVE to be an expert on the matter. A little research can go a long way. Imagine how much a lot of research can take you.
Let's use armor as an example:
At one of the writer's conferences, I went to a panel about medieval weapons and armor.
The entire class was AMAZING and in that short hour, I learned a ton! One thing I learned was that Armor is a lot more powerful than I once believed. In fact, in Lord of the Rings in the last major battle, the Orcs run through and kill thousands of soldiers. But guess what? The Orcs didn't stand a chance and the Men's armor (in real life) would have been a lot stronger. The Orcs had hardly any armor and very basic weapons. There is NO WAY they could have penetrated the armor of the men. See? I now know that for when I write epic battle scenes.
And even more importantly, because I learned that, it led to a lot more research. I'm still not an expert, but I know enough to write a believable battle scene. (Well, at least enough about the armor.) :)
Write what you know and Know what you write.