I need some planning advise

Okay, I am taking two weeks to visit Glacier National Park and Yellowstone with my wife. My question is, how can you make hotel reservations when you are not sure of your arrival times. I don't know exactly when we will be through with Glacier and start heading to Yellowstone. Also, is mid-June a good time to go. I have had several people tell me it's best because it is less crowded.. on the other hand many roads in the Glacier area may still be closed. As of now we are planning on being there in mid-June. And are National Park Cards worth the money, or should you just pay park admissions as you go? Sorry for all the questions but I am a first time (long haul) road tripper. :)

Thanks for all your suggestions!
Tim

Tim,

I'm going to Glacier and Yellowstone this summer (August) from the UK.

We will book (reserve) motels along the way based on our assumed itinerary. We will pay particular attention to the cancellation conditions for each motel. You can usually pre book and then dynamically re-schedule as your plans change en-route as long as you know if you need to let the motel know the same day before 16.00 or the previous day before 16.00 (the two most common cut off in my experience).

UK