I have a 2007 Honda Odyssey with leather seats.
What can I do to keep them from drying out and cracking?
Clean leather seats with a good leather cleaner and follow up with a good leather conditioner. I personally like the leather honey series of products, but that's just my opinion and a little off-topic.
More on topic is how often this should be done. A good rule of thumb is every three months, but your exact needs may vary based on many factors regarding how rough you are on the leather, including sun exposure, transporting pets, etc. If you live in Texas or Arizona, or somewhere like that with lots of sun exposure, and you're taking your German shepards with you everywhere, you may need to clean and condition every month. Or if you're talking about a car that is parked in a garage at home and at work, and you're very gentle on the leather, wear only the softest cashmere pants, and never allow animals with sharp claws to even drool around your car, then maybe you could get away with conditioning only once a year.
Still, three months is a good baseline to start out with and then adjust from there if you feel you need to. Once you start using a quality conditioner, you'll feel the difference in the leather, and you'll be able to start to tell when it needs reapplied because the leather will start to stiffen up and feel dryer.