Bring lots of water, you can refill the bottles when you stop. It's easy to get dehydrated riding long distances in the summer. On a hot day try to drink a bottle of water every hour.
Also, pack some light long sleeve shirts, jeans, lightweight gloves, bandanas, etc. Or riding gear. You want your skin covered on the ride, unless you wanna get there looking like a lobster! Also, believe it or not on a hot day you'll actually be cooler in long sleeves/gloves/etc. On a bike, the wind dries your sweat too quickly for it to cool you. Covering your skin prevents that and allows your body to effectively cool itself. (Of course it still needs to be breathable)
Going to montana, make sure you have some rain gear, warm clothes/chaps, that sort of thing. Ya never know, the weather can turn!
A Passport wouldn't hurt either if you decided to venture a little north!