1. Trace articles back to their origin. For instance, if you're reading an internet post about a certain time period, find the references at the bottom and go backwards from there. If you can find the original book/article, you'll usually get more complete and accurate information.
2. Find pictures. Like they say, a picture is worth 1,000 words. It helps to get a visual in your head of the world you're trying to describe. Even a modern-day photo can help.
3. Start with the basics. Research governmental structure, organization of towns, current political situation, role of religion, types of housing, foods eaten, and layout of the landscape. This is when I usually write my first draft. I get a rudimentary framework of plot, characters, etc, without too much emphasis on the historical setting.
4. When you write your first draft, make sure the dialogue doesn't contain any modern phrases (like the word "okay"). Research language and speech style for your particular time period. Sometimes you won't be able to find anything on this, but if you can (specific phrases, colloquialisms, terms of formal address, etc) then you can go back through during your first round of revisions and tweak dialogue here and there.
5. Details. I usually devote an entire round of revisions to historical detail. Working scene by scene, I research the little stuff (the Irish term for stove, the exact method for threshing barley, types of plows, names of specific pieces of clothing, who gets to wash their hands first before dinner, etc). These additions are what really bring your story to life. For me, it's best to focus on these things after I've got a first draft, because otherwise there are just too many terms and details to remember. Also, I don't want to disrupt the flow of work on my first draft to go look things up. I often mark specific pages that need more historical detail so I can come back to them later.
6. Remember, small additions can make a big difference. References to political situations, customs, and cultural nuances help ground your story in the real world and provide an additional layer of depth.
So there you have it. I still have a lot to learn about writing historical fantasy, but I hope to improve with each book. How about you guys? Does anyone else write historical? What are your tips?