I will be transporting the second-hand Linea Mini I recently bought in Seattle to my home in Denver. The car will be some sort of mid-size or intermediate rental and I will have plenty of room in the back seat and in the trunk. I do not have the original box and packing materials for the machine.

Any suggestions on what I should do to prepare the machine for the trip besides removing all "removable" parts?

Have it stored safely in the car - use seatbelts and/or other belts to strap it securely. I once transported an old lever over 400 miles in the trunk, all went well. Until I drove away from a traffic light stop too fast, already back in my home town. The machine tilted a little and it resulted in a bented sheet metal - I could have cried...

And, well, prevent the remaining water from freezing, so no prolongued stops in high altitudes

I'd also suggest to put the machine in/on a waterproof blanket in case something gets loose... so you don't have to explain any water marks to the rental guys

Hava a safe trip!

My machine was shipped from Chris Coffee (across the country) as follows: The machine was in a tightly fitting factory box, then that box was surrounded by 3 or 4 inch foam and placed in another very heavy cardboard box -- and there were additional foam blocks stuck to the bottom of the outer box to support the inner one. I would be inclined to do something like that - double box with foam in between -- even for a car trip. I'd put it in the trunk, and of course transport right side up!

Not a whole lot to it, wrap something non scratch around it like a soft blanket, and make sure, absolutely sure, that it cannot tip over in any direction and you should be fine. If you want you can drain the boilers, they have actual 1/4 turn ball valve drains if you desire. If it has aftermarket high legs on it, remove them or replace with the stock, the high legs have too much lever to them

Thank you for the suggestions. I won't have the space or the tools to drain the boilers. So, if the overnight temperature gets too low, I'll take the machine into the hotel room at night. And I'll make sure things are well-padded during the day.

Drain the boilers. Water leaks are really not fun, and considering the 3.5 L steam boiler, it will be substantially lighter for you to move into the car. Sloshing water in a boiler makes it much harder to carry, and you're most likely to damage in when getting it into position.

Get a buddy to help spot or lift. The machine weighs 70+ lbs and you can easily throw your back out with a bulky 70 lb object. Get some moving blankets from Home Depot or Uhaul. Strap it in, tight. Most seatbelts will ratchet when you pull it all the way out, use that. Pad things and double strap. The machine is secure and not prone to flying around in the car when you can shake the machine, and the car moves but the machine is still stuck still in the seat. When driving, give yourself much more room to brake. Turning and acceleration are relatively mild, panic brakes will cause things to fly.

My recent experience loading an espresso machine...

A travel case might be helpful if you are planning on moving the machine a couple of times.

Audio Utility Case with Wheels and Cubed Foam
http://www.youtube.com/watch?v=KT8pp_cwkOs