I just cut the ribs a little extra long. When I bend them, I pull them up between the gunnels and clamp them there while they cool. Then it's pretty trivial to mark them and cut them just where you want them. Also leaving them long gives you a little extra flexibility to adjust them to match fair curves for the keel and stringers.

I think when I made my last kayak I just layed the rib across the gunnel and added 2-3 handwidths to that. I also set the keel before doing any of the ribs so I would be able to shape the rocker correctly. I noticed things worked better if I did all the bending in a single session, and all the cutting afterwords. It was harder to keep the chines and keel fair when I bent a few ribs, cut them, bent a few more, etc. I did need a lot more clamps, but spring clips work fine and are cheap enough.

I know certain types of bending forms require you to stick the end of the rib into the form in which case you wouldn't be able to use this technique. With fairly good quality, well-soaked ribs I don't usually break too many ribs freestyling it, but I'm thinking about using pieces of steel tape (like the kind they use to bundle up lumber) as a backing strap clamped at either end of the rib to prevent breakage even further (without having to resort to a bending form).