I've used a piece of small cable,(aircraft cable),inside of tubing before to help control kinking,although not in an aircraft application. Find something a little smaller than the I.D.,insert in your tubing,lube well,and bend around an appropriate sized mandrel for your desired coil or bend. Remove the cable afterwards,(you did leave a tail hanging out to grab it by,didn't you?),and you're set.
I didn't read all the replies, so I hope I don't repeat what others have said. I was frenching some antennas into my Olds, and had an idea about tacking on a coil of brake line to top off the tubes for the antennas. Well, I just took the 1 1/2 tubing I was using and started to "Roll" the line around it, went around a few times and slid the "coil" off the tube. Quick, dirty and looked good......hope this helps
To Straighten- uncoil the coil as much as possible by hand, clamp one end to a solid object, the other end to your tractor and floor it. A natural gas pipeline is not a solid object.
IN '65 I bolted a coffee can to my inner fender well on my 442 then had my fuel line coiled in a smaller dia. and had it running into and out of the coffee can that i had packed with ice. Idea was to keep the fuel cool before it went into the carb. Cooler fuel atomized better,so I was told by my neighbor, who was an old stock car racer transplant. Don't know how much it helped but it sure looked cool
The ends of a barbell set with the screw in plate holders work well for making tight coils. It gives you a nice groove to follow when you are coiling the line.