You have to out a red flag or you can be ticketed, here in Texas because it's in the dot code book for items sticking out 1.5 or 2 foot past the back. Might be the same for you but it does also let people have a reference point so they don't impale themselves.
Put some weight on the other end as mentioned. Or, you can but those removable ladder racks that go into the stake pockets in the bed
Definitely use a red flag on the end (or flasher/light if it's after dark). A sandbag on the cab-end of the ladder will work wonders, but you can easily tie it down to the lower bed cleats, if you don't have weight to put on it. Loop it around the first or second rung of the ladder and use a taught line hitch to tie it, so you can take the slack out and keep it tight after tying. Run a rope over the tailgate end to keep it fast to the tailgate, as well. If you're having trouble keeping enough tension on the cab-end rope to keep the ladder from bouncing, try running a third rope from the rear lower bed cleats to the second or third rung of the ladder, and tie it so the two ropes on that end of the ladder are in tension to one another and it should keep bouncing down to a minimum. Obviously this is an "around town" kind of solution, as hauling said ladder in that situation for 100mi or something is not ideal. You definitely want weight if you're going longer distances than just one house to another in town.