I don't teach any classes in my field of research. The closest I come is a four-field course on environmental theory and anthropology. I don't even teach the introductory archaeology course at Tiny Liberal Arts College (it's taught by Dr. Mr. Palimpsest). Instead, I teach introductory courses in biological anthropology and (gasp!) cultural anthropology, as well as higher-level courses in bioanth and archaeology. The only higher-level archaeology class that I teach is focused on a region I've never been to, much less conducted research on.*
I can easily spend 6 hours prepping for each class period, especially if it's a new class. Since I have a high teaching load, if I'm not careful I lose all my time to teaching and have none left over for research. One trick I've learned that helps: do all class prep in the evening, after the kids go to bed. This has two advantages: 1) it forces me to work during a time that I often get lazy and just want to read a book (or write a blog post). The panic of not having anything to teach the next morning will work wonders; and 2) it forces me to limit the time I devote to class prep. Prep-work, like a gas, expands to take all available time between its initiation and the actual class. If I prep in the evenings, then pure need for sleep will eventually force me to call it quits.
I should note that this plan didn't work at all when I had all new preps, my first year on the job. Then I had to work evenings and all day to get it done. I will also admit that this plan worked a lot better before Boo Too was born. Between sleep deprivation, and an infant who will only sleep while in my arms, my evenings have been increasingly devoted to the Red Menace (aka Netflix). I've found the evening prep time to work well in the past, however, and highly recommend it. I hope to return to it, once Boo Too starts sleeping on his own.
*"Why?" you ask. Don't ask. The inner workings of this department are not for the faint of heart.