Keep in mind that in its factory form everything in the fuel system between the tanks and the lift pump is under vacuum and not pressure, so even if a clamp does get loose the hose will not slip off and spill fuel all over the place. So you can go single clamp on a smooth line and have no problems at all. But if you must:
You know how to make a double flare? Start making one, halfway thru the first step (where you use the die) stop. You should end up w/ a nice slight bubble at the end of the line, small enough to easily slip the hose over it yet still large enough to hold it in place good. Make sure you don't complete step one of the double-flaring process all the way, cause then you still have a bubble flare but now it's quite fat and its edge is kinda sharp too (or at least not very rounded), neither of which is good for the hose to slip over.
Edit: Oh yeah, use either transmission cooler (SAE J1532) hose or fuel injection (SAE 30R9) hose, the regular fuel/emissions (SAE J30R7) hose works w/ diesel but dries out and develops cracks faster than the other two. The 30R9 stuff is supposedly the cat's meow, but it also costs accordingly.
