I came up with this years ago, and am suprised that few seem to know about it. Very simple, but takes a lot of words to explain:
Sucking on the hose to start a siphon is a good way to get a mouth full of gasoline or worse. My way ovoids any chance of this.
It requires a second hose, which can be a fairly small diameter, and fairly short. In practice I usually have some automotive vacuum hose around, and that works fine. It only needs to reach from your mouth to the opening of the fuel tank. You also need a rag or some such.
Insert the siphon hose into the tank so it reaches the liquid. Put the other end into the catch container. As with all siphons, the outlet end of the siphon hose needs to be lower than the inlet end.
Now place the second hose just into the tank. It need not (and best not) reach the liquid. Now wrap the rag tightly around and between both hoses and stuff it all into the tank opening such that a fairly air-tight seal is formed. It won’t be perfect, and it doesn’t need to be.
Now blow into the short hose. This will pressurize the tank enough to start a siphon in the long tube. Once the siphon starts, you will need to remove the rag so a vacuum doesn’t form in the tank and stop the siphon. The open short tube will do this, but if it is small, it will slow the siphoning.
A big tank, partly empty will probably require more than one breath to get enough pressure to start the siphon. When you stop blowing to take the next breath, the pressure in the tank will blow gas fumes out the short tube and make you gag. (and it is not healthy to inhale such) With very little practice, you can prevent this by sealing the end of the tube with your tongue while you draw the next breath. Much harder to explain than to do, and smaller tubing makes it easier.
One other thing I have learned the hard way, and this may be peculiar to Fords, since those are the only cars I have siphoned from:
The fuel filler neck is not a single tube. There is a concentric rubber flexible tube inside it that is the actual fill path. The steel outer tube allows air to come out of the tank as fuel is fed into the tank via the central tube. If you are aggressive with inserting a siphon tube, it is easy to detach the upper end of the inner-tube and push it into the tank. This will cause fueling problems, as the pump will keep shutting off when the now single tube needs to “burp”. Fixing this is a royal PITA, which may require buying a new filler neck and/or dropping the tank for access.
So don’t use a very stiff siphon tube, like the PE irrigation tube I found this out with. Use a flexible siphon tube and finesse it into the tank rather than forcing it.